I don't really understand your point either, both starcraft (when I'm not mistaken) and for example Emperor: Battle for Dune have certain levels inside spaceships, in a closed environment. It works great in those games, but are clearly meant as, although important missions, mini-games or gimmicks. People just don't like the limits these closed environments give (both limiting the developer but especially the player). Personally I loved those special missions of Emperor: Battle for Dune, and I think the Dungeon Keeper series are still part of the better strategy games out there. Having said that it's good possible when designed properly. On the other hand it's kinda the hype at the moment to make large outdoor areas (like Ground Control I or II), and honestly I like those environments too.
Don't forget RTS is all about large battles conquering the enemies territory, and in that case outdoor bigscale terrains are the place to be, if you want to make those battles seem epic. Btw, when I think about a game like Battle for Middle-earth I and Warhammer 40k: Dawn of War, then I think the scale of those terrains actually aren't really amazingly large, but do give the kinda epic feeling anyways.

Cheers