Well, you could go ahead and compute the costs of your dishes, water, electricity for heating and lights, rent, etc., and prorate them, and add a valuation of your time, and figure out what your home-cooked meal is really costing you. And then the difference would be less.
There are some things that would be a huge bother to fix at home. If you want just one portion of, let’s say, lasagne or shepherd’s pie or moussaka, you can’t just cook that amount—you have to make the whole thing.
It can be hard to buy certain ingredients in single-portion quantities, so either you’re going to waste some or you have to commit to a lot of use of that ingredient.
You probably don’t have a hot enough oven for fresh pizza. You can’t bake just one roll or one slice of pie. And you need a giant pot to cook a lobster.
So there are some things it just makes more sense to have prepared elsewhere. And for that convenience, the service, and the other expenses of the provider (plus a profit, or else why are they in business?), you have to pay more than the mere cost of the ingredients, as others have explained.