I suspect a significant part of this is that no PC floppy drives ever incorporated the disk detection and electrical inject/eject. That and IIRC the 800K drives vary the speed of the spindle to maintain a constant linear velocity. By the time you added all the bits needed to make a PC floppy drive work properly on a Mac, you wouldn't be saving any money over using a real Mac drive and you'd still have a lame manual eject drive.
Doesn't seem to be a great shortage of Mac floppy drives yet though. They're fairly robust and generally easy to repair. There may be some market for a floppy interface SSD, but I suspect a SCSI SSD would cover a far greater range of needs.