Because once everyone starts doing it that in itself will cause a fuel shortage; not to mention the serious problems caused by longs queues of cars at every garage. Also the maximum amount of petrol you're allowed to store in a domestic garage is 20 litres in two 10 litre metal containers. That's only about a third of an average fuel tank. It isn't even certain there's actually going to be a strike yet - talks are due to be held with Acas - and even if one is called 7 days notice of it will have to be given.