Different classes have different curves, obviously. But they're still pretty unforgiving after a while, and it takes lots of intense grinding and effort after that point, and even a single death can take away enough experience that it becomes too much of a pain to keep leveling. And by the time people get that high up, they're also generally old enough that if they haven't hacked it as an adventurer up to that point, they won't be making enough money from adventuring to support themselves, so they need to get a job, which is less time to go level up, making adventuring harder, and so on.