A lot of it is stolen, stripped of what the theives want, and then dumped to get rid of evidence. Whatever is left anyway. The rest is simply because it's cheaper than bringing it to the landfill. Landfill is $12-15 for a truckload. The fine is (up to) $10000 for illegal dumping where I am. Lots of risk, but the likelihood of getting caught at night is so low that it isn't really a factor. Landfill really just needs to be free for individual residents. The amount the gov spends on cleanups is probably more than their $12.