The usual response I hear is "companies know people won't fill rebates out and it saves them money." If companies are using a promotion to gain customers and build relationships (keys 4 and 5 of The Seven Keys to Marketing Genius) then pissing people off is not a good idea.
Look at this pile of crap my wife was trying to fill out. She was scurrying around a pile of boxes and nearly cut her pinky off with an exacto knife trying to remove a bar code. All the while thinking bad things about the people that were making her do this.
Even if you can not give as much of a discount- people would prefer the promotional discount up front over these cat and mouse shenanigans.