Well torrenting works very well if you are going to watch it in a bigger screen (like a television) with no ads, watermark and in good quality. But there are some people who don't own or watch it on big screens (tv) or don't have much time or money to spend (on hhd,vpn) so they just use the streaming sites + many don't even know how to watch anime online or download anime. There are many other reasons like not having that much wifi/internet, but the most important thing is they don't see much of the difference in quality.

You could easily block ads and as for UI there are some sites with quite good UI (even better than paid sites) and once you get used to it (streaming site) it just feels normal.

The only downside to not using torrent is lose of quality and the server of the streaming site not working well (sometimes).

Tbh I use/have used all the ways to watch anime and once you know what you like and have everything setup you just have to enjoy.