And the US did launch a converted stage in the 70s with Skylab (albeit, Skylab was built on Earth and didn't contribute propellant / thrust... a rather different beast ;) ). That is, a dry workshop rather than a wet one.
To a rocket scientist, it's "obvious"; to a habitat designer, it's a nightmare. They're designed for dramatically different needs, and in-space construction is very difficult (and thus expensive). Orbital habitats are not just big shells, they're complex structures that take a lot of work to make. The original proponent of the wet workshop concept, George Mueller (who had worked with Von Braun on the idea), himself had switched to arguing for a dry workshop over a wet one by 1969 (this eventually became Skylab), telling congress that the wet concept had become just an inferior stopgap based on necessity rather that desirability.
There's this concept that launch costs are everything. They're not. A lot of times, it really is just cheaper to spend more in launch costs than to do more engineering, assembly, and/or in-orbit work.