SE would also have to consider how many other projects would have to halted to put people on a FFVII remake. It's essentially the same undertaking that would be required to make a brand new FF game, only without the guarantee that it would sell the 5 million that's expected from a new entry in the series. Remakes rarely outsell their originals, and given how Final Fantasy has lost some of the brand power it once had, it's not a sure thing that it would even top FFXIII's 5 million -- and that was a multiplatform release.
If you aim lower, on a platform like Vita, you're going to lose some of the 'wow' factor that people would expect from a remake. It would cost a lot less, but you would also probably turn off a good chunk of potential buyers. It's not as big of a deal when it's on a portable. There's no real solution for them with a FFVII remake, which is why it's understandable that it hasn't happened yet. If a big company like Sony came forward to help fund the development, then maybe it would happen, but I doubt we'll see it from SE alone any time soon. They're not the most efficiently run company anymore.
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Simplicity is the ultimate sophistication.