"The Browser" has turned out to be a pretty terrible application API, IMO. First, which browser? They are all (and have been) slightly different in infuriating ways going all the way back to IE6 and prior. Also, a lot of compromises were made while organically evolving what was supposed to be "a system for displaying and linking between text pages" into a cross-platform application and system API. The web's HTML/CSS roots are a heavy ball and chain for applications to carry around.

It would have been great if browsers remained lightweight html/image/hyperlink displayers, and something separate emerged as an actual cross-platform API, but history is what it is.