first, and foremost, mortgages are secured against a physical piece of property that will almost always retain a significant portion of the original value. in the recession, homes might have lost 60-80% of their value, but they all bounced back reasonably over time. in contrast, a vehicle loses a huge portion of its value immediately and by the time a car is 20 years old, its not even worth repossessing.
second, homes can't move. you have a much better chance of repossessing a house than you do of repoing a car.
finally, just for fun. your claim that people are less likely to default on a car loan because they need to work is taken out of context at best, or possibly is just plain incorrect considering the prevelance of ride sharing tools and public transit. People only need to keep their cars for work.... if they have jobs.