People that become necromancers were obesessed with their own mortality. They are afraid of dying. They didn't become necromancers for power. You are mixing your own interests for Necromancy for theirs. Apparently the gods don't think it is a good idea to give Necromancy Slabs to Power Hungry people.
Just consider the Necromancers as nerds afraid of dying, locking themselves in towers to be even safer and trying to "live" on the outside world through their books.
Not necessarily. Maybe that's the motives behind the original necromancers in a world, but once people see the power they wield along with immortality, naturally there's a chance someone is going to try learning necromancy solely for the the power to raise the dead.
Remember that necromancy isn't the only flavor of immortality out their. Someone who doesn't want to die could also try becoming a vampire or werebeast, or whatever other crazy secrets a modder has put in. There would need to be a specific reason to choose necromancy over anything else, even something as simple as not wanting to feed on the blood of others or turn into an uncontrollable beast every full moon. As inconvenient as those sound, some people might look past the drawbacks and go that way over necromancy for their immortality needs. Raising the dead is necromancy's selling point, and that should influence if and why people choose to learn it.
Also, I wouldn't call the gods of Dwarf Fortress very responsible. I wouldn't call them infallible either. Their punishment for desecrating a temple is turning the offender into an immortal monster that hurts other people that weren't involved in the crime. Giving secret knowledge to power hungry people isn't out of the question, and might just be their way of entertaining themselves.
Even if avoiding death was their original intention, that doesn't have to be the limit of a necromancer's ambitions. Once a necromancer doesn't have to worry about dying anymore, they can start to understand the scope of their ability to raise the dead. Realizing they can defy the laws of nature, they'll probably want to put their newfound power to raise the dead to use. To do that, they have to go out into the world to find corpses. That forces the necormancer to interact with the world and involve itself with whatever is going on at the time.
And if the necromancer's one and only concern is never, ever dying, that just makes lichdom all the more appealing to them. A lich is very difficult to put down for good, their body is as durable as any other undead and their soul can just hide in its phylactery if it's destroyed. All it has to do is hide the phylactery in the tower and the lich can go out and do whatever it pleases.