FSI is one thing. Market absorption is another.
Absorption for 360 West was very low when it was under construction cuz of such high ticket sizes (45-48Cr). So Oberoi had to deploy a lot of their own cash to finish it. And they've taken a very long time too.
So right now, even after the building is ready, about 63% is unsold.
Taking it to it's full height would mean putting in a lot more money, more supply, which the market wouldnt absorb. Thus, he's have to deploy more of his own cash.
It's not easy.