I dont think people have, as a rule, been crazy bulk buying. The supermarket was very busy this morning but most trolley's were very reasonably stocked.
The shops stock for normal demand. If every household buys three more tins of beans, an extra pack of paracetamol and a new pack of toilet rolls 2 weeks before they would otherwise have done, the system is going to struggle.
When ''everyone" is telling people they will/should be locked in their houses within days what would you expect people to do?