The stores with mannequinns aren't in the mannequinn making business. It's not like these places get them for free, they have to pay for them. Are females going to donate money to these stores in order to pay for the wide variety of mannequinns that would be needed? Also what about the females that *do* have body types similar to the current mannequinns?
Since if I owned a store I'd say f*ck it, no more mannequinns at all. You want to know how you'd look in an outfit? Try it on and look in the mirror.
Since before you said this wasn't unreasonable, but you don't seem to want to get into the specifics of how they'd accomplish this without somehow offending women. The only way to avoid that would be to literally create a mannequinn for every single body shape.