Because it's a miracle it even works on consoles, it was slapped together and rushed around until things just made sense, unlike other Rockstar titles from that generation it uses a subset build of the engine, to clear it up, here's how RAGE is broken is broken down...



RAGE ERB1

- Rockstar Presents Table Tennis



RAGE ERB2

- Grand Theft Auto IV

- Midnight Club LA

- Grand Theft Auto: Episodes from Liberty City



RAGE ERB3

- Max Payne 3

- Grand Theft Auto V/Online



RAGE is developed by Rockstar San Diego, the studio that handles the Midnight Club and Red Dead series, for Red Dead Redemption though it uses neither ERB2 or ERB3, it's like a highly customised split off version of ERB2 mixed with some RB3 features (think of it like a 2.5), The code for that was a complete mess and changes were made daily without being documented during development, and as such no PC version of the code was ever made as they strictly focused on getting the Xbox 360 (which was the main focus of RAGE during ERB1 and ERB2) and PlayStation 3 version of the game stable.



After Red Dead Redemption, Rockstar used the year in which L.A.Noire was being pushed (which doesn't use RAGE) to heavily rework RAGE for a new build which took advantage of both PC and PlayStation 3 features, as well as pushing the Xbox 360 to it's limits, this resulted in ERB3 which debuted with Max Payne 3, which of course had a brilliant PC version compared to the previous PC release which at the time was Grand Theft Auto IV/Episodes from Liberty City.



The ONLY way Rockstar could do a remaster of Red Dead Redemption is if they took the game itself and transfered it over to ERB3 and THEN ported it to PC (which in turn would allow PS4 and XBO versions too), but to salvage that mess of a code and actually put that amount of work into it would cost a lot of man hours.



It's not because "they just hate PC gamers", etc, or anything silly like that, it really is because the game itself is a coded mess and it's a surprise it got stable on 360/PS3, if you look at the assets yourself you can even see things like audio and textures thrown in last minute not packed in the archives properly, etc, as an example of how things were being thrown around right up til gold with the game.