While a stolen bike probably isn't that valuable, what matters is that in most places, there's basically no chances that you will be caught. While thieves aren't always the most rational people, they are rational enough to know that a low-paying risk-free crime can pay a lot if you do it enough times to compensate for the low value of each stolen booty.
And I'm not kidding about the absence of enforcement and low risk of stealing bikes. Here's a NYT journalist filming himself stealing his own bike in New York City to see how people who see him will react. You gotta see this to believe it. He even uses powertools and does it near cops: