You're welcome. I'm always happy to help.
I'm not sure it's a Marvel thing specifically but more a movie thing. The benefit of comics and TV shows as well, is that they can approach the story telling from a serialized format and take the necessary time to develop certain characters. MJ's been in the Spidey books for a long time--her development has occurred over decades. And for movies, for the most part, it's not a serialized approach even with a continuation of characters and plots--most story elements are neatly wrapped up in about 2 hours with some threads carrying over to the next film, often with time jumps, etc. And with such a small amount of time on the screen that makes drawing out a more detailed development of a supporting character a difficult thing to do. So, for the sake of time and convenience you get characters introduced that only go through whatever development is necessary to move the story along. See MJ's characterization in the Raimi films as a prime example. MJ ended up having a few of her comic traits but was more an amalgamation of several of the women from the Spider-Man comics--Gwen and Liz Allen, mostly. IMO, this was done just for the convenience of making a movie and needing to have one specific love interest. I'm not trying to be critical of the process however that's what was done and what you get is a character named Mary Jane but she's not at all accurate to 616.
All that said, I still don't understand why they keep shying away from presenting a solid, comic accurate portrayal of MJ in the movies. Especially now that Marvel is making movies that have more on-going threads and connections should or could allow for better development of even the supporting characters...imo of course. But I also think it goes back to what I was saying earlier, for some reason the current crop of writers for movies and even the comics seem to have this notion that the 'perfect' love interest for Peter is one that mirrors his character, i.e. is book smart, into science, is withdrawn or introverted, has other similar interests, etc. MJ is definitely in the 'opposites attract' dynamic and imo with her specifically and how they help develop one another, she is by far the most perfect foil to Peter. I don't think another love interest could come along and supplant her.