The simplest answer is that the air in the cabinet forms part of the suspension.
Before box speakers everything was pretty much close to open air/open baffle designs.
Physics Question: Why does cabinet volume matter so much to bass response?
If you put the same 8' woofer into a bookshelf speaker or a floorstander, you will get a different frequency response. Does anyone know what's happening with the air pressure on the inside of the cabinet to cause this to happen? Does the woofer in the bigger cabinet have greater excursion, and therefore produce more amplitude?