Easyjet have a legal obligation to have cabin crew in the aircraft - minimum of 1 per 50 passengers. Of course, the cabin crew need to be paid for the duration of the flight.
While seatbelt signs are off, these cabin crew are largely free for whatever airline management want them to do - i.e. they are effectively people whose wages have already been paid. Thus getting them to flog overpriced food / drink to a captive audience incurs little extra cost. Very little of the food will expire that day - so wastage is minimal. The only real cost is flying the trolley and its culinary contents.
Do passengers really need a bacon baguette on such a short flight ? Of course not - but it's still a way to make a bit more money out of customers.