Because they want to. FFI to VI are already remade. And what reason they have to create so many spin-offs(FFVII, FFX, FFXII)? I don't know the reason, but the fact is that SE do it all the time.
And i don't know if they are remaking it right now in secret, but the possibility of a remake in the future is big. They did that with the past games(I-VI) and the next number after VI is VII. And remember we are talking about the most popular game of the FF series. AND a remake could fix all the retcons and plot-holes created in the story by the Compilation.
The fact that SE already said that FFVII would take five years to be remade, and if they do it they want Nomura to be the director, is a prove that SE is thinking about it.
In my opinion, the FFVII remake is just a mater of time.
And i understand that people stating that FFVII is being remade in secret, all the time, is annoying. But people stating that "FFVII will never be remade" like a fact, is much more annoying, and just prove how some people are ignorant about FFVII's success.