Not saying it won't happen. Once you've ported something once, subsequent ports are cheaper and the sales just keep accumulating over time. But that first hurdle takes time to clear. And with mobile the case was so obvious (a BILLION potential customers?!?!) in a way that it's not as obvious with Steam, so that's why we've been able to benefit from mobile ports.
Think of it:
Final Fantasy 3 -> DS port got ported to Mobile got ported to PC
Final Fantasy 4 -> DS port got ported to Mobile got ported to PC
Final Fantasy 5 -> Mobile port got ported to PC
Final Fantasy 6 -> Mobile port got ported to PC
Final Fantasy 7 -> Existing PC port got updated
Final Fantasy 8 -> Existing PC port got updated
Final Fantasy XIII through LR -> Brand new port, but from a source system where code was somewhat modern
Final Fantasy X/X-2 -> PS3 port got ported to PS4, both systems modern enough where it might be fairly simple to do a PC port.
FF1/2 have mobile ports, but are low interest compared to the more modern games. Conspicuously, FF9 and FF12 don't have a recent port. Although with rumours swirling around an FF12 update / port / remaster / whatever, maybe we'll get the advantage of that. 9 remains neglected. It's too bad.
9 could really really benefit from a PC port. I really loved the game but it's the FF game I replay the least because it's a bit of a slog. Battles are slow, load times are long. It's the game that could benefit the most from a lowered encounter rate, better load times, modded to reduce some of the battle effects, etc.
We'll see.