Generally, my answer to that would be "because I don't like it". However, I was making chicken-in-a-pita sandwiches for dinner last night, and wanted to add a buffalo chicken option for our guests. So I whipped up a batch of mayo, substituting hot sauce for the acid component. It took 3 minutes, used only ingredients everyone should have at home (with the possible exception of lemon, if that's the acid you're using), and probably cost less than 50 cents for a little more than a cup of mayo.
1 egg yolk (room temperature, preferably)
1 Tablespoon of water (or preferably 1 teaspoon of water, 2 teaspoons of an acid, like lemon juice)
1/2 teaspon of salt
whisk to combine then begin streaming in 1 cup of oil in a slow steady stream, while whisking constantly. That's it...really.
Sorry for the lack of pics. I was cooking for a crowd, and running late. The pitas were egged, and were wonderful. Chicken breast, butterflied, Dizzy Dust, direct grill over a bed of lava. All was received well, especially the buffalo mayo.