In a normal sofa there is a foundation which can be sinuous wire, webbing or coil springs (both drop-in or 8 way hand tied.)
A sleeper does not have any of these types of foundations. Instead it relies on the sleeper mechanism with the mattress.
If you remove the mattress the sleeper mechanism by itself is insufficient to support the cushions. The sofa/sleeper cushions will “sag” noticeably without the mattress. It will also be extremely uncomfortable.
Sleepers with the mattress and mechanism will typically feel firmer than sofas with the standard types of foundation.