Nothing is stopping you. You're free to use a desktop web app that either doesn't work or is terribly slow or has limited features on a mobile web browser. I'd love it if all apps were PWAs but that is not the case.
That's not terribly compelling. Android and iOS banking apps also don't work, are terribly slow, and have limited features.
Eh? The bank apps I’ve seen appear to be webviews wrapping their mobile site. Much to my chagrin, I’ll add.
My broker has a much more fully featured web app then it does on iOS or Android. You can do things in a browser that the phone apps will send you to a browser to do. BofA is just as good of a web app. Maybe people are just used to using their phones?