Browsers to me are amazing. I can run untrusted code in my computer from all over the world and at least with Chrome the odds of my machine getting pwned are pretty low. The Chromium team fixes security issues quickly. For that I get this amazing environment where I can see things as incredible as Google Maps 3D, games, music apps, etc all sandboxed incredibly well.
The whole thing is far far more secure than running native apps on Windows, MacOS, or Linux, they're arguably far more secure than Android and maybe more secure than iOS.
In other words, they aren't just an OS, they're a secure OS. Something most OSes don't give me.