Sure, you can buy just a burger, but the Big Mac combo meal is only $10.99... a single burger will cost you $23.
Your fries are cold? Well I see here that you ordered the basic meal.. for just $5 more you get the premium meal, which gives you a maximum fry temperature of 160 degrees. Actual temperature may be less depending on time of day and drive-thru congestion.
Oh you want ketchup? Sure, you can get that in our add-on package, which comes with ketchup, mustard, relish, vinegar and "special sauce," for just $14.99!
5 minutes after leaving the restaurant, you would get a phone call asking if you would like to come back and order dessert.
And your only alternative is to have frozen food delivered to you from space. Sure, it takes 2 weeks, and the options are limited, and most of the food you tastes like rubber, and it costs twice as much as McDonald's... but at least you have options!