To add another thing about this; not all liches are evil.
Remove all preconceptions about them, other than they're undead. First off, not all necromancers want to be liches. In fact, the choice to become one is always different. Some do it for more power, continue research, or guard X artifact/demons from getting into the hands of mortals/escaping so that the things they are guarding don't do any harm. Mannimarco became a lich just so he can continue his efforts to show that Arkay is a lie from a previous age. Lich king Barilzar become one to prevent mortals and the Tribunal from getting their hands on Barilzar's Mazed Band. As he says "You have no place here, child of living flesh. The Mazed Band must not be allowed to leave this tomb. The Band should never have existed at all. That was my folly, and this is my curse. For all eternity, I am damned to walk in this half life, to keep my creation from destroying the hearts and minds of mortals. Those who would challenge my fate will pay with their lives." Seems pretty reasonable
Spoiler , why else would Almalexia want it? She does end up abusing it when you hand it right over to her
. Then there's Aesliip who even though was kicked out of the Skaal for practicing necromancy, he tried to warn them about a bunch of daedra planning to destroy everyone in the island. He fell on deaf ears, but he still protected them by creating a barrier. But, he started to grow old, so he made himself a draugr (undead version of a nord) to make sure the barrier stays up.
Another thing, to many old mages and necromancers doing intense studies, death is extremely inconvenient. Best way to make sure you don't wind up dead and your research half complete would be to be a lich. The Telvanni also practice that, which is why many of them are well pass the age of expected living, even among altmer mages (who would be the most general long-lived race and class of people).
Also, liches are only vulnerable during the process of lichification where they need to place their soul somewhere else for the time being.