I'm not sure why you'd want them. They're not reliable outdoors, and while other containers aren't quite as big for sorting in, they work well enough. I'm assuming this is the only reason you want cupboards outside. Maybe a better discussion would be lack of size in storage containers to begin with?
Opinions expressed in this statement are the authors alone and in no way reflect on the game development values of the actual developers.
The main argument against the ability to build cupboards outside is that it would allow practically free (some stamina) additional 64 slots of hearth home capacity from anywhere in the wilderness without any upfront preparation (apart from taking an axe, saw and water flask, which I guess everyone has in his belt anyway, or craft the former two in the field).
It would lead only to a confusion that people might that to believe cupboards outside are intended. Especially enrage anybody who has no idea about outside decay and how it affects cupboards.