They did, and so have many other game developers made many other games, for many other people.
with 800 cars, at an hour each, is 800 hours of play time. Devote 20 hours each week to playing time. That means you've got 40 weeks, of play time. How many other games are released in that time? I've not stats on that, but I'll guarantee its more than 4.
Why would PD knowingly model all these extra cars when many people won't even use all of them?
Since GT4 came out, I've bought one game and had one given to me. Which puts me decidely in the minority when it comes to gaming. My wife has bought two more. Tiger Woods 10, and COD World at War. I played Burnout Paradise City twice through, and thoroughly enjoyed it. I play GT5P perhaps a total of 40 hours. COD 10 hours (maybe) and Tiger Woods Golf another 20. Now, the above stats are purely my own and not very good stats.
PD has to contend with a slew of other game titles coming out over the next few months and it would have served them much better to have released the game months ago, with multiple iterations of various cars, available as paying DLC.
Wasted time on Multiple iterations is still wasted time.
Of those 900 cars, are you going to drive them all? At 3 minutes per car, that's still 45 hours just to sample all the cars. Considering that in 3 minutes the variations in each car would hardly be notable when considering course times. Even so, the driver is more of a time decider than the slight variation in the car making a difference in lap times.
The multiple iterations of excessive cars that PD produces is, in my opinion, wasted time.
In this day and age with extremely shortened attention spans, the majority of buyers will buy the game, beat it, and sell it back to purchase something else. Nothing more than a tick mark in their belt of "Games Beaten in Year 2010."
GTPlanet is a bit of a different demographic as we tend to focus quite intently on the game. As such, it doesn't contribute to my stance that most people won't find a use for every single car.
Post them Online as DLC. Game is released with 3 versions of each car, then as months go new Versions and Manufacturers cars are released as DLC.