Cause it takes them 4 years to do it. There is a language barrier with most engines. They go with one engine per publisher instead. Capcom is the only company doing that right with 3-4 hits. UE3 has Bioshock, Batman AA, Borderlands, Gears, ME, R6V, and many more. That's one western engine. What if SquareEnix decided to release 3-4 games on an outsourced engine vs. building crystal tools for FFXIII? Good? Bad? I don't know.
Frostbite 2, CE3, and UE4 show that western devs can make another smooth transition to next-gen and then some. UE3 basically debuted at the beginning of this generation but now we have engines that can scale beyond what consoles can do with ease. Next generation consoles will still likely not be able to fully tap an engines like UE4. PC releases are proof that the UE3 engine wasn't taken complete advantage of just from the frame-rate.
Western devs have tech that is 10 years ahead. If they develop on this tech all they have to do is scale back all releases for porting. Are we saying that JPN is close since they are just "now" changing some philosophies?