Correct me if I'm wrong here, but something that renders at 720p on a console should not be a benchmark for a game that is supposed to run at 1080p on a PC.
You are wrong, mainly because you're comparing across separate platforms. You're saying that something that renders at 720p on a console shouldn't be a benchmark for something else that almost certainly renders at (at best) 720p on a console because on the PC, where the display resolution selected by the user determines what resolution the game is rendered at, the game can be rendered at 1080p. There's no comparison to be made there. If the game's made properly then it'll be possible to have it rendering at several thousand pixels by several thousand pixels with a full 60FPS on a powerful enough machine, but that's not really a solid indication of absolutely anything in terms of what the game's assets or engine should be capable of. It's just an indication of how PC games generally work.
Basically, you can't claim that they can't be compared because Skyrim has to be able to run at 1080p on PCs, because on PCs that's something that any game is expected to do given powerful enough hardware. If you want to claim that it can't be compared because one has to run at a significantly higher resolution then you need to compare the games on a similar platform (in which case the claim doesn't work, because we don't know what resolution Skyrim's going to run at and it almost certainly won't run at anything higher than 720p).
Trying to think of the best way to put this, but... PC hardware is unbound, so "it has to be able to do X resolution on PC" means absolutely nothing.
I would have assumed that internally rendering things at a lower resolution frees up resources for other things.
It does, but once again the issue is that you're comparing across multiple platforms. I could say that Skyrim is supposed to run in 2160p on PC (and it really, really should be able to, unless Bethesda's made some very weird decisions with the engine), but that doesn't tell us anything useful about it.
In RDR, the seamless landscapes and transitions from exterior to interior is a good example.
The landscapes are. The transitions aren't, since there are no transitions. Interiors in RDR are part of the exteriors, not something separate that you transition into.
Wouldn't it also affect the textures in the game? The lower resolution the game, the lower the need for high resolution textures is?
Not really. Low-resolution textures are still visibly low-resolution textures even when the game itself is running at a somewhat lower resolution - a low-res texture is going to look terrible up close even if you're running the game at 480i.
EDIT: Just in case someone reads this post and no previous post. Skyrim looks a hundred time better than RDR or any game in recent memory, I'm just talking here.
Completely disagree on RDR, for reasons I've already explained.