Because of the difference between current gen and next gen/pc architectures the porting time would be quite high. So unless Rockstar plans to release PS4/Xbox One and PC version (if at all) in 2014, they already exist.
via N4G: pc news feed http://n4g.com/news/1354786/gta-v-next-gen-and-pc-versions-should-already-exist
via N4G: pc news feed http://n4g.com/news/1354786/gta-v-next-gen-and-pc-versions-should-already-exist