Cardassian design, perhaps instead of doors there is some sort of forcefield.
Apparently, there's nothing at all - no inner doors, not even inner walls. Just railings to hold on to. Which is a bit dangerous, and apparently how Cardassians like it.
Well, okay, there's a bit
of wall there, behind the backs of the users. It's even a plot point in "Crossfire" where a falling lift can be stopped by Odo thrusting his arms against this wall segment and bulging it out against the shaft walls.
I guess the back wall is handy when the lift is traveling sideways - it prevents the occupants from being blown off the platform by the onrushing air. But it's quite clear that there is air in the shafts normally
, just like there is air in the TNG or TOS-movie shafts every time our heroes venture into those (but those are special emergencies