There's quite a few PS2 games that have been released in HD for the PS3. I'd assume they're selling well enough to justify the means.

The difference between that and a 6/7 remake for the PS3 is one of cost. All they're really doing in a HD upgrade for 10 is optimising the graphics for a higher screen resolution. For 6/7, they'd literally be starting a new game from all but scratch graphically.