Why do some people in the crowd sing for the entire concert?

It's not their fault that they are enjoying the concert! If you think they're annoying, find a new seat

Thus, the reason I do not go to concerts. People are annoying.

This is the one of the reasons I don't attend rock or rap concerts. I have nothing against people enjoying themselves, it's just not for me. So perhaps you can change seats if another is available. All of the rock concerts I've watched on TV or on film, the audience was very active by singing, yelling, waving their hands and even dancing.

Cause they are enjoying the concert

Who did you go see?
I think some people obviously are just super excited to see an artist they like so much perform live that they can't contain themselves. I completely understand your frustrations though especially if you were next to obnoxious people. That's annoying.

I've never even been to a concert so I guess my opinion may be lacking, but honestly it's probably not that uncommon. They may like singing a lot or just are just really into it. If someone wanted to just listen to the band, why not just stay home and listen to the track? If anything, the point of concerts was to be completely into it. It's about being with people who like the same artist, it's about the event, it's about the party etc, not just listening to the song.