I don't know whether the rack mounting hardware comes with new Ampeg gear -- I bought mine used, and the Ampeg owners manuals that I've seen don't do an adequate job of addressing the subject.
Compare that to Korg -- Korg ships their rack gear with the racking screws, shoulder washers, and flat washers included with the gear. They even mention in the owners manual how to install the unit using the mounting hardware that is included. Here's an example: http://www.korg.com/uploads/Support/...0892770000.pdf
Regarding the question about why most used rack gear doesn't come with the hardware -- I honestly don't know. I see an awful lot of rack gear that has been rashed up by half-baked racking methods, where people didn't use the appropriate mounting hardware. The result is that a lot of gear gets damaged unnecessarily through carelessness or ignorance.
Haven't checked the local hardware yet.