Because on a DVR, one still needs channel +/- for ... changing channels. Isn't this obvious? DVD+/-R/RW if used for TV recording has the same issue.

Even if one doesn't channel surf much, as most people stop doing after using DVRs, on Tivos they are used extensively as page up/page down. Personally I wouldn't mind additional hard buttons for page up/down, a lot of sat/cable boxes have both page up/down & chan up/down buttons, both used rather extensively.