Short answer is comics are comics, films are films.
Long answer:
-- In the comics, Peter met Mary Jane Watson face-to-face 42 issues into the series. And Mary Jane ultimately became the second most important character in the story. In a movie, every film is going to condense material. You talk of 8 Spider-Man movies in various continuities, well there are more than 1000 comics of Spider-Man in 616 alone (ASM+Spectacular+other Satellites). The Raimi movies at best cover proportionately a comics era between 1962 to the mid-80s around the time of Venom's introduction, and that's three films, and that still covers a bigger chunk of comics history than any other Spider-Man we have had onscreen. Only one of them shows Spider-Man as a married man (ITSV). You can't delay introducing the second most important character in the story just for the sake of continuity. So you have to introduce MJ early. At best you can have a first movie in a trilogy cover the Lee-Ditko era and end with Peter graduating and his break-up with Betty Brant, and meeting Gwen in college and the final scene has to end with Peter seeing MJ for the first time. That might work. But now you have the issue of casting.
-- Remember the movie Troy in 2004. Well a big issue with that film is they cast an actress (Diane Kruger) as Helen of Troy, who's the most beautiful woman in mythology, "the face that launched a thousand ships" and instead people complained that she's not hot enough. Because ultimately the introduction with MJ is the same moment. It's that moment where Peter opened a door and saw the most beautiful woman in comics at that time. And the reason for that effect is that you shifted from Ditko and Ditko-drawn characters (like Gwen) to a pure Romita image of beauty. In the comics, MJ had to compare to Kirby-drawn women (Susan Storm, Jane Foster, Janet van Dyne) which nothing against them but Kirby doesn't always draw women with idealized beauty and with Ditko dames (Betty, Liz, Gwen, Clea). So that's why she was able to pull off that dynamite introduction because she looked like nothing like any woman in Marvel at that time.
In other words, it's a moment that truly works in comics, specifically serialized monthly comics and it works for reasons that have nothing to do with storytelling, so it's impossible to adapt and would probably not contribute much if one attempted to do it. But in movies, Hollywood actresses tend to be dolled up, and unless you cast people around Peter as somewhat "ugly" and plain (which well is bad optics and not something that would fly today). If you do a movie where Betty Brant is the main romantic lead, you need to cast a proper actress but then she'd be fairly pretty for the romantic story (since you as an audience do need to feel for Peter-Betty in any hypothetical scenario) and then at the end of the movie you need to convince the audience that the new hot actress behind the door is the way to go, and it won't work. It would be hard to pull off and ultimately detrimental.
-- In the case of MJ, the serialized way her and Peter found each other, grew up and dated when both of them matured which is how Weissman wanted to do it, wouldn't work well without resorting to some kind of sitcom chicanery. What that means is you would need to cast the actors in such a way that the actress who plays Gwen gets upstaged by the actress who plays MJ. And that's incredibly hard for one thing, but also something movie agents are incredibly alert about and would be vary about casting any of their clients in such kind of roles. In the Andrew Garfield movies, it was suggested that Shailene Woodleigh be MJ to Emma Gwen and that would never have worked, not because of any fault in the actress and so on, but simply at the time Woodleigh wasn't as big a star as Emma Stone (and she still isn't). It would have hurt MJ's status as IP had they gone ahead with that and it's a good thing that MJ didn't show up in those TASM movies. You can get away with that in sitcoms. Take FRIENDS (not a great show by any means, not one that's grown well) but Ross and Rachel are obviously set up to be the main romance...that means that every relationship or romance either character has along the way has to be a false-lead and usually Friends make that by essentially making the false-lead character normal-at-first before making them bizarre. So if you do that with MJ and Gwen it wouldn't work. Weissman's Spectacular played with this by making Peter and Gwen annoying in the Ross-and-Rachel sense, i.e. this irritating non-functional couple who pine for one another while flirting and dating others, so that eventually the audience would buy that Peter would be better off with MJ who's more stable and honest.
There are so many great MJ moments I'd prefer to see instead:
-- Parallel Lives flashback for instance.
-- MJ telling Peter about her family in ASM#259.
-- Them getting married.
-- The Airport Kiss.
-- Fractions' To Have and to Hold.
To name a few.