Waiting to get a good balance of "your eggs are still reasonably healthy" and "if you haven't had kids until now, it'll probably be a while still" is probably the reason behind the current advice.
On the other hand, it may be a useful tool to resist expectations to become a mother until it becomes socially acceptable to say no. So it might be important even if the eggs are not getting used.
Then you add potential complications from anesthesia and the egg retrieval itself, and you have a net negative expected value.
The first time my wife underwent egg retrieval, the surgeon accidentally pierced her ovary. She has had pain on that ovary since.
Is this assumption based on anything? Not saying you're wrong, after all the majority of the world's population live in poor countries where people have children younger. But at least In my social circles it'd be more accurate to say 80%+ of women start having children at age 30 (or later) then are done with it. And I know multiple women who had their first child at 40+!