- Optical mice have a processor running an image processing algorithm to detect motion.
- Hard disks have a processor implementing error correction, block remapping, and SMART. SSDs and SD cards are similar.
- Pretty much any USB device has a processor to implement the USB stack - pure hardware implementations exist but are uncommon (and are little more than hard-wired CPUs).
etc, etc. Exposing all of that complexity to the CPU is not useful, and would dramatically increase the scope of software that has to be written to make anything work.
I expect reprogramming to require a JTAG board. Easily reprogrammable (via USB) peripherals will be a malware vector really quickly.
(And that's even without getting into the bandwidth issue - the camera on a modern optical mouse captures far more data than it can transfer over USB.)