The same reason you would sell your car: To try and recoup the cost that you sunk into it, to repurpose and hopefully find productive use for the capital. The trouble you will run into with the degree, with it quickly depreciating to having no remaining value as soon as you drive it off the lot, is that nobody will want to buy it from you. The car will retain at least some value with its much longer depreciation cycle and ultimately scrap value.
You sell your degree by selling your time for a higher rate, because it is generally more useful than the time that the less-educated would be selling.
While the data shows that wages stagnated through the rise of post-secondary schooling, proving that time has not been sold at a higher rate, there is some underlying merit to your theory...
Cars being the prime example. Wages were not stagnant through the rise of the automobile. People were able to sell their time at a higher rate by being able to travel further to where the higher paying work was found. Wage gains only started to taper off once car ownership reached critical mass, providing no additional market advantage.
So, yes, you are quite right that most people buy a car because it is the vehicle that gets to higher paying work, justifying the high cost of ownership. However, that is separate to our discussion about recouping the cost when you sell it. Both a degree and a car depreciate, but the degree dramatically faster.
The same reason you would sell your car: To try and recoup the cost that you sunk into it, to repurpose and hopefully find productive use for the capital. The trouble you will run into with the degree, with it quickly depreciating to having no remaining value as soon as you drive it off the lot, is that nobody will want to buy it from you. The car will retain at least some value with its much longer depreciation cycle and ultimately scrap value.