[WorldMaker]

The easiest dumb answer is that Rockstar has never licensed RAGE to an external developer and presumably doesn't develop the engine with that in mind at all so it is probably under-documented and hard to work with for any team that isn't Rockstar.

Also, I think there are other engines that do as well as RAGE, but none of them are licensed for external use either and are secret sauce to their various developers.

[Cyph0n]

I think that’s fairly self-evident, hence my inclusion of “in theory” :)

Are there other internal engines that have been used for open-world games with detail and scale at the level of GTA or RDR?

[WorldMaker]

Just off the top of my head:

Volition's multiple unnamed/unbranded engines for the Saints Row franchise.

The RenderWare fork used for Crackdown 1. The unknown engine for Crackdown 2 (presumed to be an iteration of the Crackdown 1 engine). (Crackdown 3 used Unreal Engine 4. Useful to mention for those elsewhere in this thread concerned Unreal "can't" do it.)

You can argue that BioWare has tried its raw best to push EA DICE's Frostbite engine as much as possible in that direction. DAI and MEA didn't have truly seamless open-world games, but they tried to get the biggest scale of levels that Frostbite seemed capable of and at their best felt "close". (Obviously at their worst they felt nothing at all like GTA/RDR level of detail/size.)

[Cyph0n]

Thanks for the list!