Because the computing resource, battery power, and physical volume spent on hardware Instagram integration detracts from what can be devoted to making the camera as good as possible at being a camera. You'd have to cram a smartphone in there, and everyone already has smartphones anyway.
And you can do light editing in-camera, if you want.
"Why can't download the photos directly to a phone? Why can't I charge from a micro-USB/USB-C cable?"
You can. Modern bodies have touchscreens, too.
And I don't want Nikon and Canon worrying about marketing to influencers, because influencers are a terrible market! I want them worrying about making and selling excellent cameras for the use of people who do need more than a phone camera or maybe a cheap camcorder can provide.