The browser became an app engine so app developers could escape the Wintel monopoly that was pretty tight in the 90's and 00's when the Internet became a thing. These days, it's so there can be apps outside of app stores.

There's been various attempts to build "Internet operating systems" that are little more than a browser (that's what Chrome OS was intended to be in the beginning I thought) but Windows and it's pre-internet legacies are so entrenched in PCs and corporate life that nothing's ever gonna change there until Microsoft makes the entirety of Windows an app.