I played about an hour of Rage on PC so far, and this kind of got me thinking.
I totally appreciate the iDTech5/Megatexture engine, and what it does for asset creation. I get that the file sizes are an issue with something like the 360, where you have to ship on several DVDs. And on the PS3, I understand that there are pretty severe memory and output resolution restrictions that would make anything more detailed than the current texture assets unnecessary.
But on PC, none of those restrictions really matter, save one "imaginary" restriction. I, and probably many other PC gamers, would have been totally fine with an install size of 30GB or more on PC; Hard drive space like that is cheap these days. The problem then comes down to delivering that 30+GB game to players.
That "imaginary" restriction is that PC games are still developed with DVDs in mind. Blu-ray drives are $60 on Newegg, which is probably near the relative price-point where the CD-to-DVD shift happened with PC games (around 2005, see Farcry or BF2). Digital distribution further trivializes these size restrictions. If companies like iD are building engines that are really justifying Blu-ray level storage capacities, why aren't we seeing a push toward that paradigm/medium for PC games? My guess is because most high-profile games are multiplatform PC/PS3/360 releases, so the lowest common denominator wins.