Chaosmancer
Legend
They all spend time becoming heroes and investing in their powers. That is time they are not spending developing their nation. Others without the split of attention that focus on kleading a nation will be more successful. Further, all the time they spend on the nation is time they do not have for their pursuits of power.
Why? Why is the bard more interested in becoming a hero and investing in their power than the fighter? Why is it that they care more about personal power than the running of the nation?
And, there has been talk of how this society could not last because of disparity, but how is this not worse? How is it not an even worse disparity that the vast majority of the most powerful individuals on the planet care nothing for ruling, and leave that to the plebians who have nothing better to do?
I already addressed some of this in a reply before but: It isn't that the wizard and king spend different amounts of power to maintain control - they spend different types. Your typical non-wizard leader is going to have charisma, clout, allies, or other forms of power - where a wizard that spent their life learning magic often won't. They have to replace what the mundane king provides with arcane might - which then can't be used for other purposes.
Why would a wizard noble not have clout, allies, wealth and other forms of power? You keep making this claim, but you aren't backing it up with anything. If the wizard is born as the fourth generation of a noble lineage, whose family helped found the kingdom, why would they have zero allies, zero clout, zero wealth? Does teaching magic automatically erase your family's history and accomplishments?
Think of it this way: A wizard can decide between ruling a nation and making magic items. They don't have time for both. If they rule a nation, they get a lot of headaches. if they make the magic item, it takes time and money - but it makes them more powerful without all of the drawbacks of subjects.
It is all about the idea of opportunity cost.
Nobles and kings can absolutely have hobbies. They can still make things, they just can't focus on it. And that isn't really a problem. Most of the best magic items that a ruling noble would be interested in only take a few weeks to make anyways. Also, magical items are generally permanent, so they get passsed down from parent to child to grandchild.
Again, your perspective is "You know, I could rule the nation, as is my birthright (because noble) but... you know... it's a lot of work. I've got better things to do."
I think this is an absolutely horrible answer to the question "why aren't mages in charge" because it says that they absolutely could be, they just see running a nation as beneath them and leave it to the mundanes.