I got a strange thing in my head just trying to mind it: we have Halo 5 Forge on PC that’s playable, BUT we can’t play Halo 5. I have only why question, why and how did you come to this idea?
H5 Forge was brought PC as a tool to make forging easier. It was never intended to be the full H5 game.
the most probably is that Halo 5 will be in PC in the future