Lanefan
Victoria Rules
To the former: a very good and valid question. (for those extremely rare times a PC is in fact encountered in its home baseThen why don't players have lair actions? Why don't monsters have Vancian spell slots?

To the latter: if a monster can cast spells then in my game anyway it either uses the same (sub-)systems the PCs do or there is an ironclad in-fiction explanation for why it does not.
Whole new worlds of design perhaps, leading to a bunch of whole new worlds where I for one am not the least bit interested in playing any characters.This enforcement of perfect mirror consistency between GM-run entities and player-run entities is precisely what forces such awful design. Ditch that seriously flawed requirement, and whole new worlds of design become available to you.
If you don't have that mirror consistency then you're intentionally singling the PCs out as special snowflakes, not representative of their populations in the setting...and, technically, also irreplaceable when they die as you've already detemrined that nobody else in the setting has the abilities of a PC.
If the PCs are all aliens from another world then this works (other than the can't-be-replaced piece, which means you're really playing in hardcore mode!); other than that, it's IMO an awful way to run a setting.