Because that's how installing software works on Windows. You install a new program which can open JPGs, it's not allowed to grab control of all JPGs unilaterally. Instead it registers itself as a handler of JPGs. Next time you try to open one, Windows prompts you "you have a new program which can do X, what do you want to use?" and you get to keep your old one, or choose the new one, once or every time.
Get a new browser installed, prompt for your choice of browser. This is an improvement to "get Edge installed, have Edge take over everything silently" and an improvement on "I installed a thing and it doesn't do anything, where is it", and an improvement on "IE 11 for eternity".
after the edge update, when i started my computer, edge opened itself and forced me through a "first run" mini tutorial thing that i couldnt skip or ignore. it wasn't even a "you have a new program" prompt, it was a "YOU ARE USING EDGE NOW".
one of the updates most certainly was not just a mime handler update. They pushed a new edge (when they swapped to chromium) and rather than leave people with old edge as their browser, they just flat out set new edge as your browser without checking what you had otherwise, and set it to auto run on your next boot.
Get a new browser installed, prompt for your choice of browser. This is an improvement to "get Edge installed, have Edge take over everything silently" and an improvement on "I installed a thing and it doesn't do anything, where is it", and an improvement on "IE 11 for eternity".