As an only single player guy, the answer to this is pretty easy.
Because many of us wants to replicate real football as best as they can, and against another random opponent online, well it's hard because most of the times people cares more about the win than that, and exploits unrealistic tactics and/or just what's op at the moment. Not that there is anything wrong with it, but it's not my cup of tea since I want to see more something adhering to a proper match, and it's much much easier achieving this against the cpu.
When, or more likely IF (probably never) somebody will make a football game that will force users to play realistically then I'll be all over it, in the meantime if I'll feel any need to play a meta and what's op at the moment I'll reinstall League of Legends

.
p.s. again, since what I just said can be easily misunderstood, that's not to criticize who plays online, I'm sure the experience could be fun and deep especially if you play against a similar minded friend. But I think it's undeniable that searching realism against a random online opponent it's still harder than adjusting your slides/difficult level offline and play at whatever rhythm you feel like. And this applies to both games of course.