Why does this not happen more often??
The 434MHz license exempt allocation that car key fobs work on, is shared with many other services and systems. Amateur radio, telemetry systems, house wireless alarms... The list goes on. So I am surprised that these problems don't occur more often.
Being license exempt (under UK law) means that no radio license is needed to operate on it, but also that no protection is given in law. It's the same situation as the WLAN bands.
The recovery/breakdown companies are aware of the problems. Their only cure is to tow the affected vehicle away from the interfering signals and try opening the car again.
As already mentioned, you don't have to make jamming equipment. Reasonably powerful transmitters are available on eBay cheaply, as walki-talkies, that will (illegally) tune onto the key fob frequencies.
You can also buy handheld radios that operate directly on these shared keyfob channels, quite legally.
In the UK, key fobs were on the 418MHz allocation, which was less prone to interference. But OFCOM deprecated that in favour of the EU allocation at 434MHz.
I suspect that this incident is far less likely to be a of criminal intent. As what would be gained by having lots of angry people milling around the car park. It's more likely to be a school boy prank, or just unintentional interference from another legal user of that same spectrum.