Except the OS's don't even ship a consistent enough browser engine. (Standards be damned.) So apps ship their own browser over and over again.
Go figure what would happen when Chrome gets shipped everywhere, because writing browser agnostic code is too hard.
There's also stuff like lorca which shells out to an installed Chromium browser.
But I think even there it’s possible to encounter platform-dependent bugs - starting from view issues (you know, different OS can use different browser engines) and up to those related to system APIs.