There may be some technical reasons for it, but I'm not convinced. The chin is a core part of the imac design language. Without the chin it would just be a display and not recognizably different from any other display. The chin is part of the design that that makes it recognizably an imac.

They didn't always have the chin. Imac G4 put the computer at the base. Consider the size of a mac mini you could do something like that today without needing half a sphere like the g4.