Because I don't have a single, specific setting in mind.
"Medieval times" type setting, with kingdoms, kings, queens, peasants, merchants, nobles, etc etc. Monsters roaming the land. Heroes to oppose them. Etc etc. It's a very very broad theme. Deliberately.
Fate works best if you've got something concrete. Out of the box, it works ok, but as soon as you've got something concrete, you can adjust the system to it. Like the DFRPG does for the Dresden Files. All the powers etc. are not part of standard Fate, it's specifically designed to model the Dresden world.
And the same thing applies here. The broader you make your system, the less interesting it will be.
For example, lets take magic. If you say "magic works like this", magic will be like that in every world you want to play with the system. System and world are fairly closely interwoven in Fate, and the way you model things mechanically always has an impact on the reality of the world you play in. Granted, that's true for other systems as well, but the flexibility of Fate allows you to do a lot of different stuff, where other systems are often locked in to their world.
So looking at the DF magic system, it does a few things. It ensures that wizards can be incredibly powerful. It also assures that wizards can't throw out too many spells at once at any given time, because they tire out. If you use the DF system, that will be true for your high fantasy worlds, where it might not always be so.
Alternatively, you could have a world where pretty much everyone can do magic to some degree, or where wizards use magic at every corner, albeit not their most powerful tricks. An archer shooting an arrow would be not much different from a wizard firing a magic missile. A thief picking a lock would not be much different from a wizard hexing it open. So the wizard could simply use guns and burglary (or whichever skills are used for the tasks, Core is a bit different in the skill list), and you're done. He wouldn't be able to do everything, his choice of skills would be the way he specialized on his magic, so that's not an issue either. The only real difference to other characters would be the way you describe the actions. Since he's just as effective as any other character, you don't need a mechanic to limit their spellcasting either. What you could do is give them a mechanic to do more powerful spells and limit that, if you really wanted to.
And there's tons of other ways to do magic. In fact, there's tons of ways to do magic better than DF. All of them based on a different assumption and philosophy on how magic is supposed to work in that particular world.
