In an XP-driven world, you don't get a choice. Why? Because Uncle Fizban wants you.
Powerful nations would draft promising teenagers and put them through "adventurer boot camp"--standardized dungeon crawls, fine-tuned through centuries of trial and error to maximize the number of high-level adventurers that emerge. It's a lot like the boot camps used by real-world militaries since the dawn of warfare, only cranked up to 11 billion and capable of producing spellcasters as well as soldiers.
The nations that don't do this will quickly be overwhelmed by the super-soldier armies of the nations that do.
Question: Why do these soldiers stay loyal? Answer: No ruler can stay in power without their support, so they get lavishly rewarded and courted by all political factions. It's a pretty sweet deal. The conscripts who died in training might not think so, of course, but they don't get a say.