Because nobody's paying Rockstar to port GTAV to PC, and I can name two reasons for Valve to make such a transaction
1. Steam Machine - Valve titles aren't enough to carry a Linux-based game console, and if Valve wants this thing to sell, they're going to need to buy PC and Linux ports of console games. Perhaps GTAV is a reasonable start?
2. Half-Life 3 - At this point, I think it's safe to say that Valve are getting out of their way to delay their flagship title, and are pretty much scraping the bottom of the barrel at this point. We can fill that barrel.