When the spelling-differences are so acute, why the languages of 2 different countries UK/USA are same:ENGLISH?

The spelling differences only seem to you to be acute; actually they are rather benign. Try sounding the similar, but different spellings, and you get a small, if any, difference. I realize that in , especially oriental languages, the slightest change in utterance of a sound can give the sound a different meaning. Not so true in English, where contextual usage has more control over meaning that does the sounding of a word.
It's hardly possible, to sound out the word "colour" without getting the same sound as that in "color". Americans pronounce the words "centre" and "center" the same ( "centre" is a French spelling, however, and would be pronounced in Quebec or Paris as "sawn-tr")
America's emancipation from Britain gave us a chance to make changes in spelling of words, and the writers of dictionaries had a ball!

Because the US was originally a British colony (well, it was called British America and was made up of 13 British colonies). That's why Americans speak English. Then, throughout the years, the Americans have standardized their language and adopting new rules/grammar/vocabulary/etc. that is different from the British English.

Because a person born and raised in America can go to England and understand the language of the native people without difficulty, and vice versa.

This is not true for all related languages. A person from Spain cannot go to France without experiencing language barrier problems.

Even though the spelling is slightly different the pronunciation and usage is essentially the same... thus allowing those that are American or British to be able to talk to each other with ease with possible confusion only being in the regional slang.

It is because Americans were pioneers in founding a new land, and through the centuries have become a very bold and innovative people. They wanted to be different, in as many aspects as possible, in language, voltage specifications, use of miles, gallons, billion(9 zeroes instead of the British 12).

The spelling of some words are shorter and easier to remember, and their accent is quite standard and easier to understand. I hope more complicated and long words in the English Language can be simplified, so more world citizens can be motivated to take up the language. Such as, double c's, m's, l's, s's could be reduced to single ones.

When one spells wrongly, people tend to have a low opinion of him, it's so sad! Grammar(esp tenses) and some complicated sentence structures have deterred many people from taking up English, towards perfection.

Many world languages like Mandarin and Japanese have a modern and simplified form of writing, making communication faster. One way to survive in a fast-paced world. Still, it is one of the most beautiful and highly respected languages in the world, and has a great potential to endure for a few centuries more, i hope! :-)))

Actually, those differences are minor and don't change the pronunciation or meaning of any of the words you listed. There certainly isn't enough there to make a case that the 2 should even be considered different dialects, much less different languages.

Who says they are? One is American English and the other is the Queen's English.