The patrons casually showing up at camp can create some odd scenarios, too. If Mizora wants Karlach dead, why doesn't she just kill her herself given the chance? Or are we supposed to believe a greater devil is afraid of a level 4 Barbarian because she's a legendary infernal general badass? Maybe we can stack up some barrels and just blow Mizora up, and Larian will have some elaborate contingency plan for that, too?
Because for most outsiders it is not that easy or allowed to go into our realm of being.
And when we see them here, it's usually just a projection of their avatar.
Plus, Devils in particular have tons of rules.
A lot of things they can't do and have to use agents.