Because having Mary Jane as "the friend " takes the emphasis off of romance as they have suggested wanting, but still allows them to explore Peter and MJ's relationship. If they introduce MJ or any other love interest, for that matter, the way you've suggested, she'd have to remain in the background perpetually or go through some type of courtship with Peter(meeting, falling for each other, the girl being disappointed by Peter's constant absences, strain on relationship, the girl potentially discovering Peter's identity, strain on relationship...wash, rinse, repeat), which again is something that's already been explored in previous films. You'd just be changing up the women/minor plot details. And I don't want Peter Parker turned into James Bond with his harem. It's already done enough in the comics.
And Mary Jane is, without a doubt, the most significant and developed Spider-Man love interest. Sorry, if you don't like it, but it's not about setting her up as "the one" (even though she pretty much is), it's about exploring MJ and her dynamic with Peter (that of being the best friend and confidant which she has been in the 616 universe more than anyone else) and not relegating a complex female character to being Peter's endgame by having her show up at the last moment just to say the jackpot line.
I think ultimately there has to be a medium in terms of the whole "love interest" thing. The reality is ,Feige wants the focus to shift more towards the idea of a teen dealing with powers and aspects which weren't explored by the Sony films. As a result, the romantic stuff is likely gonna take a backseat regardless , if the MCU films are an indication of how they would handle it.
We may get the MJ or Gwen characters again, but I think fans assume too much that they have to build up to "the one" for Peter , or that they
have to play out "The Night Gwen Stacy Died" storyline again.
Looking at it, not as a fan, but instead as Filmmaker or storyteller who have may have a certain arc for the character, I could see Marvel not considering the Mj and Gwen characters priorities , for the first film, especially if the character is in the context of a civil war /infinity war reality in which its about the challenges of being a hero more than the challenges of his love life.
That's not to say that the films couldn't or wouldn't feature MJ in some capacity , but I think to assume that the filmmakers
need to plant the seeds for MJ or Gwen for that matter, and
need to establish who he's destined to be with , i think, is mistaken. Marvel is clearly taking a different approach entirely in their approach toward the franchise than Sony did.
Sony's emphasis was the romantic relationships effects on Peter , where Marvel and Feige seem to place emphasis on the effects Spiderman has on Peter's teen life and social life which is a much less narrow concentration, in addition to Peter being exposed to a much larger universe. Now , no doubt MJ and Gwen could be included in that social circle and they could be "friends" who eventually develop into something more, though I wouldn't assume that Marvel/Sony wouldn't exploit characters like Liz Allen and Felicia Hardy instead. Sony has certainly spent years trying to have the films focus on the latter. SM2's original script included Felicia Hardy, the original SM4 script was to feature Hardy, and she clearly was going to have a bigger role in the ASM films and S6.
It would make sense if Sony and Marvel decided to focus on her in the first film than to go with MJ and Gwen. Again, not as a fan , but as a filmmaker who wants to tell a fresh story and not be burdened by the baggage of the previous franchises, I could see them giving the GA a new female character in Peter's inner circle as opposed to focusing or alluding to the future which the GA has already seen two times in a row. I'm just speculating but ultimately I think as fans we have to prepare ourselves for Marvel to approach the story in a very different way then Sony did, including characters which haven't been exploited but are classic parts of the Spiderman myth.