Well actually they do, and the price difference in the second hand market is around £400 to £500 depending on dealer or private sale. The extra it costs when new isn't reflected in second hand value and that is true of many cars.
Essentially, the difference between a fully loaded car and one that isn't is that one sells easier than the other and the buyer will only add a token value to the fully loaded model. There is an argument to made in that people buying second-hand cars probably have more consideration for resale value than people buying new cars - lets face it, no one buys new cars with the hope of getting anything like the purchase price back, unless it's something REALLY special.
Phil
Like Sarnie says, have to disagree, more loaded cars sell for more - at pretty much all levels. I recently spent 2 months looking at tens of Ford Focus's in the £2k price bracket, could have snapped up a decent 1.6LX for £1.8k no problem but hung out and bought a similar mile/condition Ghia for £2.2k - I paid for the premium kit, and that was a cheaper one that I had to wait 2 months to find and buy the day it went up for sale.
When I was looking to buy a Zed, spent about 4 weeks doing it, noticed in that time non GTs sitting on autotrader longer than GTs, but in my view they were somewhat overpriced so whether they were there because they were non GTs or because they were overpriced is open to debate.
Zeds suffer from seasonality as well, after I bought mine prices of Black GTs went up and I was dead chuffed with my bargain - now though the price (taking into account 9 months of time passed) is way down on what I paid because we are in winter. So you are at a good time to buy I think. By all accounts it happens each year from what I have read on here.