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!
All constants are variables.