My guess is PS3 limitations or they had a minimum PC spec to adhere to based on global market share.
The thing is they can always outsource this sort of work, Bethesda do it all the time. My guess is they are too proud to hire outsiders to do their work and I bet their assets are a mess to dig through so no-one on the team can be bothered.
The thing is the shimmering is the number 1 complaint, it is literally the games weakest link so I think it must be addressed if they want to improve anything.