The fact that you'd need a flat-bed to steal the bicycles is a security feature. People steal bikes because the penalty isn't serious. Who is going to risk a felony and their tow truck license for just a few bikes?
Average bike on the street? Do you have any source of similar claim?
The realizable value of stolen property is vastly less than the new price, even when stolen new. More like 1/10th.
But these are very seldom expensive bikes. The bad news is that your $75 bike you think is too cheap to steal from the train station rack is exactly what this kind of bike thief is looking for. They want low risk and fast turnover.
The kind of Orwellian things society would need to do to prevent these kinds of crimes of opportunity would cost far more than the crimes themselves cost anyway.