The usual reason keyboards,mice and disks aren't include is that the computers were bought at auction from a school,government or large corporation.
Typically the cpu boxes are stacked on pallets,the keyboards piled into a d container(large cardboard box the size of a pallet,the mice in another container,books and disks either sent to landfill or another conainer,AND in the past few years, the hard drives either reforatted or removed ,depending on the level of security(paranoia) of the entity disposing of the systems.Goodwill now sells PCs with hard drives formatted,and so do many small shops.I have been told Microsoft threatened the small dealers reselling used computers because the OS on those computers was licensed only to the original user? and Microsoft wants another license fee if Joe Average buys the ex-Fortune 500 machine.This seems to vary as some resellers claim entire P3 systems with licensed XP available at their shops for under $100.(Tempting).
Cincinati,Ohio public schools some 10 or 12 years ago disposed of hundreds of C64 Commodores in this manner.No one could buy just a ,or even a dozen full systems,.You were stuck with bidding on huge stacks of monitors,disk drives,cpus,and power supplies seperately.Most went to the landfill unsold.And even worse,having trashed all those C monitors they bought TV/monitors so they had something to hook up the VCRs.(AAAAAAAAAAHHHHH!)
Info courtesy of a friend on the inside of a typically clueless public school administration.