I agree that you can't just look at the cost of the basic (uncooked) food from a supermarket and consider it unquestionably a rip-off.
Staff time/wages, VAT, pitch fee, equipment, ingredients, fuel, profit/living - these are very considerable costs, so why wouldn't you expect them all to be free or maybe just a pound or two?
There have always been people who have complained about a cafe charging even £1 (or whatever) for a cup of tea that they could have made at home for pennies. The obvious answer is that it's entirely your choice and either bring your own/an alternative or go without.
The fact that you aren't at home and able to access your own kitchen should maybe give you a clue as to at least some of the added value.
You could say the same about taxis: you could use your own (bought, taxed, MoT'd, insured, maintained, fuelled) car and drive and park it yourself, so why ever would you expect to pay a lot more for somebody else to facilitate it all for you?