Well, sure, inverting a binary tree isn't hard to learn. But there's countless interview questions, not just that one. A lot are more tricky - and not everyone is good at coming up with such solutions from first principles under interview pressure.

While I'm reasonably good at these style of questions, I'd be very unsurprised if there were a few that'd just catch me cold, even today. And I'm probably working in a field that's a lot more day-to-day data structure heavy than most (RDBMS internals).