Reasons have been given , but they have been either not read or not understood. Potential shortages of some foods, for instance, could happen because without trade agreements on imports from the countries of origin of those foods, there would be paperwork to be checked at the places where they entered the UK. Checking paperwork needs a human eye, a human intelligence, and a certain amount of time.
While one consignment of food was being checked through customs, the lorry behind it would have to wait its turn, so would the lorries behind that - whether their contents came from an EU country, one on the European continent that wasn't in the EU, a container ship from the Far East or one from the US or South America. Many, many consignments arrive at our ports and airports every day! It could take hours - days? - for transport to clear the queue. Some produce can't be held up like that without going off.
The paperwork on imported goods, whether food or not, could include a bill for customs duty. That would be added to the price, resulting in higher prices.
The other things that could and very likely would follow our leaving the EU trade agreements have similar explanations. It is not rocket science to work out the mechanisms which could cause delays, inconveniences, costs and shortages. When they happen, those affected will blame "the government" for them and for not passing legislation to stop them happening!