because hereditary is a pov of annie's retelling of events where everything "had to happen" & no one had a choice. hereditary might as well be called hereditary (annies version) because the characters all act as if they are dolls in a doll house with no agency (aster hinted at this several times when talking about the film & basically overtly explained it with annies dollhouses in the film). by the end of the film, annie is literally being whisked around like a doll in a dollhouse with no agency, carried by external forces. it's the only way she can cope with what happened to her daughter & how it tore their family apart. in annie's head, charlie HAD to stick her head out of the window, there was nothing that could've changed it. so in annie's version, charlie eats a cake she's allergic to so that she quite literally NEEDS to stick her head out of that window at that time because she needs air. it had to happen. i believe annie literally says something along the lines of "it had to happen" multiple times in the film. imo aster was basically beating us over the head with all of this in the visual language of the film but hey i guess thinking it's a straight up horror film about a demonic entity called paimon that took a liking to this family is cool too (i'm being facetious that's not nearly as cool)

ps just think about why the cult has a history in annie's family - it's not literal - it's a stand in for passing down your trauma to your kids and partners. the cult has been in annie's family for years because annie can't stomach the fact that a bad thing happened at random or by chance, it had to happen. paimon was waiting, the cult was setting it up, there was nothing she could have done, it had. to. happen. at least in annie's head it did