These guys tracked down an expert car thief and got him to explain how he can steal cars in under 30 seconds.
tl;dr: Most modern cars contain well-known software vulnerabilities that are accessible through the OBD2 connector. Unscrupulous Chinese manufacturers have cataloged and automated these exploits using tablets and OBD2 adapters, and you can buy these tools freely and legally online. You may need a blank key fob from the right manufacturer, but the app literally automates the job.
Car makers have been shipping vulnerable software for decades without consequence (car owners and dealerships end up paying for theft, not manufacturers), and they continue to do so. Only Ford appears to be improving their security.