For kids, it's hard because it must be interesting and not too hard, mix some interesting math concept and some hidden part that makes it an interesting puzzle.
For a Millenium-like problem it's harder, because it must be interesting so mathematicians care about it, it must be hard so no one can solve it for a few decases, it must have some hints that is possible to solve it in a sensible amount of time (let's say a century!). It's more like discovering a meteorite than making a super nice porcelain pot.
There are plenty of open problems, so once the current wave of solutions in done I guess we will find a few that out of reach of the current LLM but we hope they can be solved in a sensible amount of time (let's say a century!).
Also an initial, cumbersome, complex proof is the first step to a more understandable, formalized proof. AI created a formal proof of Fermat's last theorem. I'm sure the initial proof was not comprehensible to all but a small subset of mathematicians anyway.
Brute forcing a counter solve has not really changed the game as much.