I actually don't think the reason is that they are easier than other open math problems. I think it's more that they are "elementary" in the sense that the problems usually don't require a huge amount of domain knowledge to state.
The Collatz conjecture can be stated using basic arithmetic, yet LLMs have not been able to solve it.
The Collatz conjecture can be stated using basic arithmetic, yet LLMs have not been able to solve it.