But that's a choice. Humans in the Culture can change their form and body to basically be whatever they want: with some caution expressed for changes which might not leave meaningful back conversions.

The humans we see are human because they choose to be.

The thing about The Culture is the universe doesn't bend to make being human the default or particularly good: it is an old world where technology surpasses limitations in almost every way imaginable - if you choose. You can also just decide to go lava rafting, where the agreed rules are materials just barely up to task.