I'd argue the reason we expect films to have self-contained plots is just that for a very long time the financial model forced them to, i.e - studios weren't going to pay out for multiple films if they weren't sure people would pay to see them, so very few films which told a story in two parts got made. If you're not constrained by that and you're lucky enough to have the opportunity to plan out a longer arc, why wouldn't you? Especially when you're doing a comic book adaptation where self-contained escapades aren't generally the done thing in the genre.
Obviously the MCU, in my opinion at least, is an example of that being done in a boring and cynical way, but there's no real difference between what's happened with the Spiderverse films and what happened with the first Star Wars trilogy. In both cases the studio made the first one, waited to see how it did, determined it was financially viable to make a trilogy and committed, giving the filmmakers scope to split the rest of the story they wanted to tell over the two subsequent films.