The living don't know where to look for dead souls
In order to raise a soul to undeath, you must first find it. Necromancers have invented a number of different techniques to track down specific souls within the world of the dead, but none of these techniques work unless you already know how to get there—the path to take. And the only way to figure that out is to go there yourself. By dying.
Oh, some living wizards have tried to pin down the exact location of the world of the dead. They've interviewed necromancers, analyzed zombies, proposed new schemas for inter-dimensional coordinate systems...but nothing works. They're like people who have been blind from birth, trying to understand what colors are. There is simply no language for the knowledge they seek. If you've died, you just get it; if you haven't, you don't.
In theory, a necromancer who was fully restored to life would retain all their necromantic powers, since it's only the knowledge that matters and not the state of being dead. Or so some scholars think. It's not like anyone's had a chance to test the idea.
P.S. Under this framework, it's entirely plausible that souls raised by necromancers could become necromancers themselves. If you don't want to allow that, you may need to establish a separate restriction (Third Iron Law?). Or perhaps this is not an issue for social reasons (e.g. necromancers are wary of rivals, so they immediately un-raise any minions who show interest in necromancy themselves).