They steal your spare tire so they can sell it back to you. My grandpa had one stolen out of a Kmart parking lot, called the police. Cop came there, was filling out the report and said "I get about two or three of these calls here a day. We don't know who's doing it, but I know where you can go to get your tire. " It was a local junkyard. Where else would you go to get a spare tire, it's on an ugly steel wheel, nobody wants to pay for a wheel and tire to match what they have... . he had to pay $35 to get a spare, he had a pile of them about 20 deep...
Well anyways, that's why they get stolen, so they can pay for whatever habit they've gotten themselves into.