It was a little before my time, but I think the approach came from minicomputers, which were probably the main application for floppies at the time the first co

It was a little before my time, but I think the approach came from minicomputers, which were probably the main application for floppies at the time the first controller chips were designed. Minicomputer interface boards (at least the ones I'm familiar with for the pdp-11) offloaded real-time processing from the main CPU, and also provided a consistent interface to software across differing physical hardware. Also presumably the economics in a minicomputer environment tend to favor a more complex interface board vs slowing down the CPU and adding OS complexity. Note that the concept of device drivers that we have today didn't quite exist -- if you were running Unix V7 you'd need to add source to your kernel tree and recompile to add support for a new kind of device. This favored hardware that looked like some older hardware as far as the OS was concerned. That's easier to arrange with a more complex interface board.