Cheap (and even not so cheap) "Mexican" food in particular is a great opportunity for the restaurant to bulk up a little protein with lots of cheap filler like rice and beans. With a burger, you have a floor set by a certain weight of ground beef. And, yes, while I very rarely go into a McDonald's or Burger King, my sense is that they're not a particularly cheap meal. And the better "fast casual" burger chains certainly are not.

You're right of course. I'm just using protein in the sense that restaurants often do to mean x grams/ounces of meat, chicken, pork, or fish. But there are obviously other sources of protein.