True those platforms have security controls, better than what we've ever seen on desktop. Desktop also needs controls like that: to use the system tray, filesystem, hardware, etc. There's at least a work multiplier of 3 for Windows, Mac, Linux. It's just one of many things needed.

Building the web toolkit took enormous effort and it sucks in many ways (see article). Theoretically a new one could be created and could also work for desktop, but there are lots of reasons why that won't happen.