The way I always figured it was that the prevalence of RFIDs being built into every conceivable nook and cranny of your average vehicle makes long-term holding of a stolen vehicle unrealistic, and the same applies to selling a boosted car intact. If you want to keep your ill-gotten gains, you have to literally completely disassemble the thing and put it back together, which is quite the time investment.
If you want to sell it, the same applies. Best case scenario, you have to strip it down for parts, disabling every RFID you find in the process, and sell it that way. Profitable, yeah. More profitable than shadowrunning, with less chance of getting shot, in fact. But the labor and time investment involved doesn't make it a popular career choice. You may get a nicer payout owning a chop shop, but being a runner gets you funding faster. In fact, I occasionally offer my players a few runs that are just boosting cars, stealing the vehicles and delivering them to chop shops.
