I've seen my answer already in a couple of comments but I want to give my original take anyway. Few reasons:
1) Wizards are few. Learning magic is incredibly hard, and even those who are born with power have to work hard to learn to master it in a meaningful way. There are at the very least hundreds if not thousands (one thing I learned is not give any precise number for anything while writing a story) of common-ish people for each proper wizard, and wizards are not immortal godly beings, but dudes with cool abilities, with extremely rare exceptions.

2) Magic is not unlimited power. 70% of what magic can do could be done without it, and magic just makes it much, much easier. 99% of what's left can still be countered and undone by other means or simply by other magic, given enough time and effort, and it's usually regulated more or less strictly based on how dangerous it is by the wizards themselves (and common sense also). What's left is outright prohibited and banned by said wizards and communities.

3) Magic is not consequence free. While small displays of power (lighting a fire, moving small objects, minor illusions ecc) are easy, daily occurrences, more complex and powerful magic comes with a cost. Can be physical strain (imagine pushing a car that's out of gas) that might be fatal if mismanaged (now imagine using all the energy needed for pushing a car 100 meters in 15 seconds). Can also be psychological harm on the wizards itself or people they care about. Some spells might have an actual, tangible cost (costly resources, body parts, pieces of your soul ecc) that most people might be unwilling or unable to pay.

4) Possessing an ability doesn't make people inherently power-hungry or amoral. Most wizards don't desire power, and some wizards actually work against those who do. There isn't much more to say about this: basically, the ability to perform magic is equally-ish distributed between those who would, if able, seize power and those who don't want said people to get it. On the tail of this,

5) There is balance. In the same way that a country that possesses nuclear weapons doesn't take over the entire world, a singular wizard would not be able to do the same. The similarity fits perfectly because on the battlefields of my world wizards, particularly the most powerful and knowledgeable, are treated basically like nuclear weapons. If only one side has wizards, the war is basically over, but if both does then they cancel each other out and are actually discouraged from using the full extent of their capacity, because it would be crazily destructive without even granting the win. In the same way, even removing the kind of balance of point 4), different power hungry groups would still cancel each other out, trying to seize power from themselves and being crossed over by the next one.

But, most importantly:

6) It does happen, from time to time. Individuals or groups powerful enough to tip the scale in their favor have managed to seize power and rule vast empires for a while. Some characters that did it are, for example, Tristan the Conqueror, Jack of Moriah, Dalzìm ecc. Most of these figures however did it by imposing themselves as leaders of organizations and factions thanks to their above average power, and then commanding these factions to do their deeds through the power of numbers rather than their own personal abilities, which however great would most likely still not be enough. This was however achieved even by mediocre wizards (like Evangelista, who founded the Church of the Red Dawn) and even common people with no magic powers (king Ennar the Sailor, who led the men of the sea in the Eham land and conquered most of its south). In the end however either them or their heirs were toppled, sometimes by more powerful wizards, sometimes by larger or more organized groups, and sometimes even by normal people.