Bikes are pretty valuable, pretty easy to steal/transport and pretty hard to identify or prove/disprove ownership. If you don't know the serial number and don't have any custom parts, how can you tell that it's your bike and not some other of the same model?

There's a roaring trade in 2nd hand bikes of all values, some of which are presumably stolen. Most of it is opportunistic to steal something easy and sell it for $100 online, though some of it is targeted because some people won't ask many questions if a bike is being sold for a fraction of it's real value.