While the OP may have inadvertently committed a "No True Scotsman!", I think he's nonetheless correct: I think experience often comes with a better appreciation of what's out there, what's still unknown to you. Before, you might have taken X somewhat for granted, but if you delve into and research X, you learn it's a complex interaction of A, B and C; now you have 3 things you're taking for granted / need to learn. A fair number of people have felt stupider after learning, which is of course the opposite of how it should be. Imagine having never seen the inside of a modern car hood, and you open it for the first time. Perhaps all you need to do is "change the battery" (simple, right? You've changed the battery in other things before) and upon seeing the inside of the hood for the first time, one might reasonably be overwhelmed by the amount of stuff crammed inside there.

Questioning your own assumptions, I think, falls out of repeatedly learning that often things are not simple, and the experience making your own mistakes and getting burned: it teaches you when to proceed (you don't want the project to get bogged down with "analysis paralysis"), but with caution and the knowledge that you made an assumption. (Whereas a less experienced person might not realize they made the assumption at all.)

I would also point to the Impostor Syndrome[1] as a sort-of evidence of that, though it's certainly possible for someone to be an expert and not feel that way.

[1]: https://en.wikipedia.org/wiki/Impostor_syndrome