I think its that most animal bodies are horizontal and thus float in a way that makes it trivial to keep their heads above water; in contrast to humans which have a bodyplan that requires constant stabilization to stay vertical, both on land and in water. It's the same reason baby animals can usually start walking minutes after being born while human babies require months to learn how to stand and walk in a stable manner

Some species like proboscis monkeys and crab-eating macaques know how to swim instinctively but most primates need to learn either through mimicry or trial and error. A lot of them try to avoid deep water even when they can swim (that's probably more a function of predator avoidance though).

I don't think humans are really bad at swimming, we just panic and splash around instead of kicking our feet like any other animal does.

Human babies can swim, kinda. But the instinctive version of swimming that comes preinstalled in our brains is not like riding a bike. It's something that we forget how to do if we don't practice, and then we have to re-learn it.

> What makes people so bad at swimming? Our big heads?

I never learned and never found the time to. But I'm pretty good at video games :)