Part of it is that whilst Geb was an evil man, he's also a lot more complicated than Tar Baphon's endless hunger (probably quite literally, in this case- Tar Baphon's knowledge of necromancy comes from Zutha's Cenotaph, and therefore is sin magic, attuned to gluttony, at least until he surpassed Zutha).
Geb was ammoral, and he did do evil things. But Geb as an undead nation isn't entirely his fault- it was a joint effort. Nex was the one who started the war (he cut off Geb, the country,'s access to northern trade). Geb certainly raised the undead who populate the place, which made him alignment-chart-evil, but notably, Nex was the one who killed those who were raised. Geb raising the undead saved both nations from starvation (ironically, the Nexians in particular, as Geb had wish-crafted their nation into a barren desert, so they couldn't survive without the combination of Geb's cheap labour and hunger-free work force providing free food).
Then with Nex vanished after an attack, Geb was depressed enough to kill himself. But his soul couldn't pass on without knowing what happened to Nex, even though he very much does know what happened to Nex- he attacked quantium with poisonous fog, and Nex managed to stumble through a portal before promptly keeling over and dying. It's a mystery who's solution was obvious within a few weeks. As a result, there are two effects on his current state- firstly, he's harder to kill than Tar Baphon, because whilst the Tyrant theoretically has a phylactery somewhere, there likely isn't any proof in existence that would get Geb to accept he won. But secondly, and this is the bit people seem to overlook, Geb's isolationism isn't a product of choice. He's still a ghost- he might be able to move about more than some, but it's pretty obvious that he can't leave Geb (not least because he really, really, really wants to go north to quantium and find out what happened to Nex).