Maybe, but there are fictional examples. Lex Luthor. Guy could cure cancer on a dare. Is one of the smartest men in his world, has vast resources, basically owns the greatest city in the (arguably) greatest nation. Can figure out alien technology. Became Presiden of the United States just to annoy his rival. But still wants to rule the world.There is a basic reason mages don't take over:
They don't need too.
Any mage that makes it to a bit of power (roughly 10th level) they can live a happy, full life with just magic. Lots of spells can provide a home, food, water, and nearly any item they want. With near zero effort and a finger wiggle they can have just about whatever they want.
Plenty of spells can make a mage a tons of money, more then enough to break any economy. More then enough money for a mage to buy nearly whatever they want.
And with even a tiny bit of effort, they can live like a king of kings.
And with some effort, they can even transcend life and become immortal.
So why even take over a nation? What does it get the mage? They can control people all day and night, but it won't get them anything more then magic does.....and in fact, mundane things are of a far lesser quality.
And, again with magic, a mage can make more money then exists in the whole nation (and, ahem, world) so it's beyond pointless to rule a nation to get rich.
So with magic meeting every need, a mage could waste time "ruling" sure....if they want to waste time telling people what to do for no benefit.
Maybe some mages would want to rule for the "fun" or such......but it's a bit pointless.
Victor von Doom is probably even smarter than Lex. A genius polymath and sorcerer, he rules his own personal country. But also seeks to rule the world- simply because he figures it would do better under his watch than anyone else's.
Just because you have vast power doesn't mean you don't have a desire for more, or that you wouldn't want to rule a nation, even if not directly (ie, being the power behind the throne, having a puppet king, etc.).
I mean, in the Forgotten Realms, this is Thay's bag, they totally want to dominate the world, despite being arguably the most organized group of wizards around! And let's not even get started on the Netherese!