Because pyside and PyQT suck in their own way. I mean sure if you only know python, or need python for other stuff, they are fine I guess.
I've had a rather big project (well, a proof of concept turned into a real project, the usual haha) built with pyside6 and looking back, I'd much rather have used electron/ts/JS. But we needed python for other stuff so I guess it made sense at the time.
Note this is beyond the capabilities of web extensions, you'd also need the user to install a cooperating native program.
At which point people are just going to make the native program an electron UI and bypass the browser entirely.