The answer is simply - the vehicle OEM's designed their transmitter/receiver to operate at close range, but with high gain equipment its possible to receive the signal at much greater ranges.
Remember the bad guys kit isn't about "normal" operability, its about gaming the technology.
I've not seen the kit myself, but I suspect it even has a function to enable drive by scanning for likely targets - so you can cruise a street and ID a likely target without having to walk up driveways and check for a keyless vehicle by looking in the window...
The scuzzbags doing this aren't technically savvy btw - the clever people are the ones building this stuff. In the same way as you don't understand how your mobile phone is designed or constructed, they are just users of off the shelf technology for a purpose.