MP4 videos can be literally 10x smaller than a GIF and better quality, so a lot easier to upload on a slow connection.
MP4 tech is obviously better because of more sophisticated compression. When I say GIFs are smaller, I mean, when one is referring to a GIF, they are referring to something small, like a short animated clip. You are being overly technical here.
I think one part that is being overlooked is that (for the creator) GIFs are easier to edit and handle. MP4s are not, they require video editing software, figuring out export settings for optimal quality/size. You have to do similar with GIF but its easier to manage/handle. I've even loaded GIFs into photoshop or aseprite to edit or remove any frame I want.
Often when creating ANY animated content that I'm attempting to share, I'm trying to maximize playtime/size/quality - you can't just share any mp4 there are limits. So it may not be an issue if Animate exports the most efficient settings. but if I wanted to edit it or change the export size, or cut a few frames, or delete every other frame to speed the whole thing up.... it can be done with video editing software but definitely not as easy as a GIF. And often when you think "GIF" you think shorter/smaller (in frame size) clip anyhow.
Also GIFs (tiny ones) are excellent for embedding in emails/presentations/slides. Those tiny <30 frame GIFs will probably never go away. There has to be a diminishing returns at some point. Here's an honest question, what is more efficient a 3 frame GIF or those same 3 frames in MP4 format? (I'm actually asking as I dont know).