The problem with remastering PS1 games is that they can't reuse hardly anything from them. In most HD remakes all they do is adjust the resolution, expand the mesh for player characters and major enemies, and use hd textures on everything else. However, the polygon count in PS1 games is so low that merely adjusting the textures would look awful on any game, be it the blocky FF7 or the less so FF9. In order for them to make an HD version of any PS1 game, they would have to completely remake the game or it would look simply awful. Redoing textures is much simpler than creating all new meshes for every enemy and repainting all the backgrounds and redoing all the magic, and basically recreating all of that as a new game. FF12 has poly counts equivilent to the average current gen game up to about last year, making it a far simpler and far more attractive remaster than any of the PS1 games.
When they say remaking 7 is impossible, what they are really saying is that completely rebuilding the game from scratch would tie up most of the companies resources as they would have to make it perfect to satisfy gamers. While the end result would most likely be FF7 with graphics as good as Advent Children, it would take them far too long for it to be feasible. After all, they are already almost bankrupt and they have to pay their employees.
That is why a few years ago SE said they wouldn't consider remaking FF7 until they had regained their glory or surpassed it. They can't afford to remake 7. But 10 and 12 only need new textures.
TL;DR
They literally cannot afford to remake 7.