Because the cost of labor and materials puts a lower bound on what can be charged for a car. Plus, most of the value in the housing markets where the bubble grew was in the land, not the structure. The value of the structure itself isn't likely much changed, and for sure its "replacement cost" for insurance purposes hasn't gone down much, if at all.
The cost of labor and materials puts a lower bound on "new construction" also in most locales (certain areas of the country excepted - you know who you are)
Personal example: Although I would like to be in the market to sell the house I have now - our local market is not too badly hit (yet?) - there's a certain price below which I'm just not gonna go. I don't "have" to move & not every seller is in a "have to" move situation either. I probably won't be asking a "new construction" $ per sq ft price - but I won't be going drastically below it, either.
Of course I realize there are a certain number of existing home sellers who "have to" sell their house (relocation, job loss, etc) for whatever price it will fetch. But if the market offers are far below what they still owe on their mortgage, well .........
Sure you can maybe pick up a foreclosure home on the cheap nowadays - but many of those (because they are foreclosures) may require significant repairs, clean-up, etc. Not every buyer is up for that. They may not be in areas you want to live in. And they are not all necessarily "bargains" for the average homebuyer who needs a house they can move right into with minimal hassle.
As to new cars - I expect the existing surplus inventory to slowly get sold over the next year or so - at what price, well that will vary, there will be some bargains & there will be some not-such-a-bargains - I fully expect in the next year both foreign & domestic auto manufacturers will downsize & slash production - so there won't be as many new cars available in coming years. (They'll still cost the same though - perhaps even more)
(What ticks me off, currently having the POV of a prospective home seller, is government programs to provide tax credits to people who buy "new construction" homes - it devalues the resale market)
Home builders expect boost from state tax credit
Reason #244 I'm now glad I didn't take that job in Cali'