Browser have shifted from document viewers to application platform quite a while ago now. I was mocking the OP because he seemed to imply that browser ARE document viewers when they're clearly more than that. Very clearly.
By the way, GMail doesn't suck. Nor does outlook365. Nor does amazon or any other e-commerce platform. Clearly they're not catalogs of online documents, so the shift might have been apparent and browsers are pretty good at it.
Also, in the wake of the NSA revelations by Snowden, I don't think anyone has any doubt that there are holes and zero-days in pretty much every stack of every OS out there. USB, Network, Browsers, Encryption libs, everywhere. Browsers are just at the top of the stack, so they get picked on more often.