I couldn't agree more. If this was a PC title I would be the first person in the mod community to get to work on updating the standard cars but alas that can't happen here.
My take on it is that for the vast majority of the production (say 3-4 years) they were intending on building all the cars to premium quality, so there would be no point in having staff working on updating old cars that were just going to be thrown away. Once it came to the crunch to get GT5 ready and the decision was made to port over the GT4 cars maybe there simply wasn't the manpower available to put them to work on them... But they could have taught the office intern how to do it and let him work his way through them lol.
Who knows what really happened. It is annoying to see something that I could rectify myself with my average 3D modeling/texturing skills though.