Why some christians don't celebrate birthdays?

Jehovah's Witnesses aren't Christian.

Read verse 8 carefully, God has not highlighted the alleged holidays, but has ordered to observe the 7th day, from when you start working, after 6 days you should not work, but pray to God.

Exodus 20.

The Ten Commandments.
20 And God spoke all these words, saying:

2 “I am the Lord your God, who brought you out of the land of Egypt, out of the house of [a]bondage.

3 “You shall have no other gods before Me.

4 “You shall not make for yourself a carved image—any likeness of anything that is in heaven above, or that is in the earth beneath, or that is in the water under the earth; 5 you shall not bow down to them nor [b]serve them. For I, the Lord your God, am a jealous God, visiting[c] the iniquity of the fathers upon the children to the third and fourth generations of those who hate Me, 6 but showing mercy to thousands, to those who love Me and keep My commandments.

7 “You shall not take the name of the Lord your God in vain, for the Lord will not hold him guiltless who takes His name in vain.

8 “Remember the Sabbath day, to keep it holy. 9 Six days you shall labor and do all your work, 10 but the seventh day is the Sabbath of the Lord your God. In it you shall do no work: you, nor your son, nor your daughter, nor your male servant, nor your female servant, nor your cattle, nor your stranger who is within your gates. 11 For in six days the Lord made the heavens and the earth, the sea, and all that is in them, and rested the seventh day. Therefore the Lord blessed the Sabbath day and hallowed it.

12 “Honor your father and your mother, that your days may be long upon the land which the Lord your God is giving you.

13 “You shall not murder.

14 “You shall not commit adultery.

15 “You shall not steal.

16 “You shall not bear false witness against your neighbor.

17 “You shall not covet your neighbor’s house; you shall not covet your neighbor’s wife, nor his male servant, nor his female servant, nor his ox, nor his donkey, nor anything that is your neighbor’s.”

I think you are a bit confussed......It's basically the non-Christian cult, Jehovah Witnesses that don't celebrate birthdays and holidays.

Again Jehovah Witnesses are NOT CHRISTIAN even though they attempt to fraudulently wrap themselves in Christian trappings to lure unsuspending people into their false teachings cult.

That is why they go door to door hoping to trick people into falling into the trap of their false religion.....they pose as Christian sheep but in reality they are Satan's wolves.

Ever hear Perry Como sing (Tell the End of Time) properly not, well that's the time that would have to expire before a JW would celebrate a birthday, of course then, no one would be around to have a birthday. 

I was born on January 1, New Years day, just another day for me. 

The early Christians, “considered the celebration of anyone’s birth to be a pagan custom.” 

The ancient Greeks, for instance, believed that each person had a protective spirit that attended the person’s birth and thereafter watched over him. 

Birthdays also have a long-standing and an intimate link with astrology and the horoscope. 

Says Ecclesiastes 7:1: A name is better than good oil, and the day of death than the day of one’s being born. 

Our name is the good reputation we have gained with God through faithful service. 

the only commemoration commanded for Christians involves, not a birth, but a death that of Jesus, whose “name” is the key to our salvation. 

The Catholic Encyclopedia says about the birthday celebrations mentioned in the Bible: Only sinners make great rejoicings over the day on which they were born.” 

I am not Catholic, just a quote from there Encyclopedia. 

It does seem like a stupid thing to do?  I mean your were born?  What is that to celebrate?  We want so bad to worship humans.  That we give them a day every year to say, "great thing I was born"  What have you done that year to deserve a party?

I celebrate Jesus' birthday. I don't like to tell people it's my birthday because I don't really like being the center of attention. It makes me uncomfortable when so many people fuss over something that seems so trivial to me. So aside from a call from my Mom, which is the person who actually shares in the event (it's the day she was able to pass me all the way through her vagina), I am glad to not celebrate. 
I don't know. Birthday celebrations seem like a childish thing to me. I remember having a sleepover when I was like 10 years old for my birthday and it was fun. But I'm not a child anymore. I can have sleepovers whenever I feel like it now. I can even invite boys and play spin the bottle if I really wanted to. 

For some, it just means you're closer to death.

Why do some Protestant denominations do or believe any of the unbiblical things the do and believe?

It's not really that important, But Christians are rude and bad people. 

Because we believe that such celebrations displease God. Although the Bible does not explicitly forbid celebrating birthdays, it does help us to reason on key features of these events and understand God’s view of them. Consider four of these aspects and related Bible principles.

Birthday celebrations have pagan roots. According to Funk & Wagnalls Standard Dictionary of Folklore, Mythology, and Legend, these celebrations originated from the belief that on a person’s birthday, “evil spirits and influences have the opportunity to attack the celebrants” and that “the presence of friends and the expression of good wishes help to protect the celebrant.” The book The Lore of Birthdays says that in ancient times, birthday records were “essential for the casting of a horoscope” based on “the mystic science of astrology.” This book adds that “birthday candles, in folk belief, are endowed with special magic for granting wishes.”

The Bible, however, condemns the use of magic, divination, spiritism, or “anything like this.” (Deuteronomy 18:14; Galatians 5:​19-​21) In fact, one reason why God condemned the ancient city of Babylon was that its inhabitants practiced astrology, which is a form of divination. (Isaiah 47:11-​15) We are not preoccupied with the roots of every custom; yet when the Scriptures give such pointed indications, we do not ignore them.

The early Christians did not celebrate birthdays. The World Book Encyclopedia says that “they considered the celebration of anyone’s birth to be a pagan custom.” The Bible shows that the apostles and others who were taught directly by Jesus established a pattern that all Christians should follow.​—2 Thessalonians 3:6.

The only commemoration that Christians are required to keep involves, not a birth, but a death​—that of Jesus. (Luke 22:17-​20) This should not be surprising, for the Bible says that “the day of death is better than the day of birth.” (Ecclesiastes 7:1) By the end of his life on earth, Jesus had made a good name with God, making the day of his death more important than the day of his birth.​—Hebrews 1:4.

The Bible never refers to a servant of God celebrating a birthday. This is not simply an oversight, for it does record two birthday celebrations by those not serving God. However, both of those events are presented in a bad light.​—Genesis 40:20-​22; Mark 6:​21-​29.