Philosophically, an OS is a virtual computer meant to be shared among programs (usually an open-ended set of programs, and usually regulating how they interact). You could say the web is not that because it's a distributed system instead of a virtual computer, so the browser makes a better match.

I guess an everything app would be different in that its maker probably goes like "Open-ended set of programs? Who cares, you nerd, I'm just gonna make deals."