Those cars are basically a huge gamble. You're saving a ton of money in buying it, but you have a decent chance of spending almost the remainder or more in repairs and some will just plain not be repairable. I knew a mechanic a while ago that supplemented his income by buying cars like that and getting them working. Since he did all his own labor and bought a bunch he always had parts to swap out, he also got a discount through his parts supplier, made quite a bit and drove a really nice car. He also had a few worthless, really nice cars sitting in his lot cause they would never run.
Oh, and most of those cars aren't lemons, most of them are cars that some kid stole and took joyriding and blew the motor or something, or it fell off the truck in transit somewhere, or got wrecked on a test drive by someone who didn't know how to drive. They start out as good cars and get wrecked in some way.