Why don't fast food Mexican restaurants have tamales on the menu?

I just had tamales for the first time, which I got from the frozen section of an Asian supermarket, of all places. I have never seen them in other supermarkets. I did a google search and found that upper end Mexican and South American restaurants carry tamales. They apparently have a long history. Tamales are not difficult to prepare. You can throw them in the microwave if you don’t have the time to steam them. At least some of us think that they make for a pretty good meal. Why then do Taco Bell and other similar restaurants not offer them?