>>96147011
>stupid monsters
This guy right here. Not necessarily stupid in a general sense, but stupidly rare to encounter organically. Roughly 2% to encounter them RAW on sylvan/faerie terrain and only there. If not for their pathetic AC they would be quite deadly.
"Stupid" also in a sense that their rarity makes them hard to use and deal with. DMs will have to find excuses to put them in their non-faerie environs. Actually intelligent flyers are also seen as "unfair" by pl*yoids, especially if their goal is mostly "abduct the weakest one and kill him in our lair".
For players it is hard to justify to have actual IC knowledge of these rare creatures. Everyone knows the Owlbear because they are hilarious and a staple, but not Perytons. Heck, most players don't even know Perytons OOC. They will be surprised at their intelligence and be baffled that these opponents will just pick up a downed guy and fly away instead of fighting to the death. And even if they are the type to look up the MM during the fight, Perytons are so unknown they would not know where to look.

tl;dr if the Demi-lich is a stupid gotcha that will likely kill you, the Peryton is also a stupid gotcha that players will actually fall for.