Why do I only gain +8 for every chess win?


Rating changes are based on the rating differences between the players and their respective rating deviation (RD) values. If you have a low RD value, due to have a sufficient number of recent games, your rating changes won't be very large, unless you beat someone a lot higher rated.
https://support.chess.com/article/210-how-do-ratings-work-on-chess-com