With Halo, the original code was safely stored and readily available. Comments in the past point to FF7's being gone, making it very difficult if not impossible to re-engineer without completely rebuilding the engine, which would be very expensive and possibly considered not worth their while - at that point, why not go the full hog for a full $60 remake? But that option is too scary/difficult in an entirely different sense, so... it's easier to just keep pounding that PS1 ISO and its PC port.
(That said, some fans HAVE been rebuilding a version of FF7's engine that reads assets & info from the original disc to make it work on newer hardware:
http://q-gears.sourceforge.net/)