Okay, you have a point there (it seems even Paper Mario and Mario & Luigi have fallen victim to that), but wouldn't the Airships count as factory levels? There are Bob-Ombs, Bullet Bills, flamethrowers, Mecha-Koopas, and bolt lifts, there aren't any factory worlds, but at least there are some levels.
Also, both factories and space are featured in Galaxy and to a lesser extent, Mario Party. I know you're just talking about the 2D games, but still(I need to stop saying that).
What I find odd about the NSMB games is the fact that there's really no reason to play as Luigi. Unlike in SMB2, SPM, and SMW(?), and Galaxy, where Luigi has some strengths and weaknesses Mario doesn't, Luigi is exactly the same as Mario in the NSMB games, as such, the only reason to play as Luigi is just preference.