Presumably they (a) don't have a genset of sufficient power (b) don't have the required hookups to disconnect from the mains and plug the generator in and maybe (c) aren't keen on running a generator next to the "switch off engine" signs.
(The nearest petrol station to me is automated, so there would be nobody to do this anyway!)
(sorry about the start of the video, but it shows the type of equipment you'd use for this sort of situation) https://www.youtube.com/watch?v=FiuNVTLHqEc