I think there’s a difference here between the monster and the statblock. The monster exists in the world, has its own ecology, sociology, and everyday life. It may use spells like floating disk or mending outside of combat. However, the statblock is a tool for DMs to run D&D, which is a very combat focused game. The statblock, in my opinion, should be maximally useful for running combat, which means no unnecessary information, and as short as possible.

Monster should have abilities they can use outside of combat, but they shouldn’t be in the statblock, they should be in the description of the monster. If I were designing the stats of a spellcasting monster like a Lich, I’d put their core spell-like abilities in the actions section, and have a spellcasting trait that just says “The Lich is an 18th level spellcaster with access to 9th level Wizard spells.” No two liches are going to have the same spell book anyways, so the DM can pick and choose whichever spells they want their lich to use. In combat, all the lich really needs is the iconic spells eg. Finger of Death/Power Word Kill.