Why aren't Transcendence and Book of Thoth looked at the same?

"Hey this guy doesn't need mana, why the fuck is he building trans?" "Wait Thoth gives you mana? I thought it was just for power"

Like they're basically the same but alot of people seem to assume that you don't build trans for power, and that Thoth is just a huge power spike mainly.

I don't see many assassins building transcendence except Loki for example, even with its power spike.