Because an operating system is a good idea, and a monolithic do-everything productivity suite subverts the point of an operating system by creating a very limited, hard-coded, non-extensible OS on top of the OS, reimplementing half of what the OS already does, but poorly, failing to interact with any components not distributed with the suite, and not allowing for any of the things modern OSes allow for (multi-display MDI support, for example, or package management.) There is no justification for having two redundant layers of OS; anything the suite does in terms of integration (for example, a clipboard implementation), could be converted to an OS feature of the parent system and used to integrate anything with anything.

And if you really think the do-everything productivity suite is the one with the right ideas and the OS you're running is the one that's screwed-up--well, then, there's still no reason to run two layers; instead, what should be done is to expand the productivity suite into an OS. Like Chromium OS and Firefox OS are currently in the process of.