Why would you bother with lichdom?

[icefractal]

For the 3.x Lich:
It's not the best form of immortality in the long run, but it may be the best you can achieve alone at 11th level, without stuff like "infinite Wishes at low level" on the table. A lot of the "casters are invulnerable" stuff comes online at 17th level (or by using methods that make 90% of the game moot). And unlike many forms, it doesn't require allies, or ongoing sources of supplies, or anything. You can pretty much ignore the rest of the world.

In most settings, the super-rapid pace of advancement for PCs isn't available for everyone, and so a given NPC Necromancer might never reach past 12th level within their normal lifespan.

In 5e:
It's a worse deal, but there also aren't as many alternatives. No way to replicate the inherent immunities AFAIK, and a phylactery is easier to guard than a clone. Plus the whole "being available earlier, and there's no guarantee you make it to higher level" thing.