Having a larger worldwide population to match with means matchmaking can do a better job of trying to find someone closer to your level. Playing any game with a high skill ceiling with IRL friends is what often just results in a skill gap too wide for either of us to have fun, and then who else can I even go play with?
And that's assuming anyone you know IRL even wants to play the niche games you love best.
OP seems to imply they're unfamiliar with the idea of playing games with other people at all, as if multiplayer games weren't a thing before the internet.