Because it was already in development for the PC which they based the PS4/XB1 versions off, and they intentionally held back the PC version another few extra months to drag out the release. Rockstar refused to do Pro/X patches for the game despite the console versions still not being close to the PC versions top settings. The game is stuck to 1080p/30 on consoles and features just the High pre-sets in most cases where as the PC version goes up to Ultra on most things and obviously can handle different resolutions, multiple MSAA methods for base graphics and reflection graphics, higher quality and detailed particle effects, etc.
IF anything, expect the "next gen" versions to possibly be the PC version on max settings with dynamic 4k and 60fps if we're feeling lucky.