Lichdom isn't necessarily an increase in power, it's a way of extending your life. Cheating mortality. There are plenty of living casters that are more powerful than the vast majority of liches - in the canon lore of FR and D&D's megaverse as a whole.
Well, immunity to diseases, cold(only resistance on 5e), attribute bonuses, magical abilities including fear aura is a huge increase in power. On pathfinder, if you apply the lich template to a creature, his CR goes up by +2 ( https://www.d20pfsrd.com/bestiary/monster-listings/templates/lich/
Even on 5e which nerfed a lot of powerful creatures, things like Legendary Resistance (3/Day), Legendary actions and etc are pretty nasty abilities ( https://roll20.net/compendium/dnd5e/Lich#content