There are a few reasons - The move to webapps meant that each app had to decide how their buttons would look like, what colors, fonts, and contrasts they’d use

There are a few reasons - The move to webapps meant that each app had to decide how their buttons would look like, what colors, fonts, and contrasts they’d use, how their workflows to do very similar things would be structured; when things were on the desktop the OS UI elements made all the decisions for you - it was the same gray buttons with black text in the same font on every app - the only difference would be whether the buttons were above or below the textbox... Another reason is that apps need to look and behave roughly like what’s currently “in trend”, as users spend the majority of their time on the big players’ apps, and not “keeping up” will make your app seem outdated and untrustworthy; ie skeuomorphism was the thing when iOS first caught on, and then everyone had to make everything flat when material design caught on...