They're herd animals basically.
But more importantly, pics of cuddly capybaras gets lots of likes and shares...
Them getting eaten rarely does, so especially when it's them and alligator, the reason is almost always the temperature or that the gator isn't hungry.
IRL they're dying constantly to pretty much anything they encounter. There's just a bunch of them because they breed like the giant rodents they are.
Like, that's pretty much their whole evolutionary strategy, reproduce so fast that predators aren't hungry enough to eat all of them...