rating only increases 8 points

Whenever I win a game I only increase 8 points

same with everybody

There's no issues with that.

https://support.chess.com/en/articles/8566476-how-do-ratings-work-on-chess-com

but why

why not ?

what an answer !

I disagree

It's because your rating stabilized after playing enough games, meaning the site is confident your skill matches your rating, and your rating progresses more slowly because of that