Because it's what people want? A better-looking version of the classic they can more easily introduce to a new and younger audience?
I'm not saying it has to be 1-1, but a rehash/remake of FF7 wouldn't need much in the way of tweaks (I do think I'm being realistic, I know it's an investment but it's a SMART investment).
I mean, as far as I can tell the handheld remakes of 3 & 4 were mostly the same with a few tweaks here and there and changes to the graphic style. In that vein, FF7 wouldn't have to go any further.
Those are handheld remakes. The problem is that no one makes JRPGs the way they were back then on consoles anymore. The closest thing you have is Ni No Kuni. I think that's the only current gen console JRPG that even has a conventional world map (I could be wrong). Would you even want to see current gen models of the chibi characters?
I could maybe see that style of remake being done for the Vita, but a console remake would have to be an entirely new game. The only things they could retain are possibly the map layouts, and the story outline.
The same goes for horror games. an RE2 remake would probably have to have RE6's controls and camera, and at that point you're designing a whole new game with the same story. In both cases it would really be more of a retelling of the old game than anything else.
I just think a lot of people are saying "I want an FFVII remake" without actually thinking about what that means.