Because it's really, really hard to actually prove that a stolen bike is stolen. First, it has to be reported stolen, which does not always happen. There also

Because it's really, really hard to actually prove that a stolen bike is stolen. First, it has to be reported stolen, which does not always happen. There also has to be some way of positively identifying the bike, like a serial number, included in the report. Bikes do have serial numbers but not many people record them.