Those 4-ingredient cookbooks are not just recipes with few ingredients - they are cookbooks especially for people who feel other cookbooks are too complicated. In other cookbooks, the link between number of ingredients and simplicity or fastness is not quite as straightforward.
I took a look at the Zuni Café cookbook to illustrate. Beef carpaccio with lemon and olive oil has only 3 ingedients, but is pretty complicated. The poleta dishes all have 2 to 4 ingredients, but are quite time consuming. On the other hand, when searching by number of ingredients, you would miss the pasta alla carbonara, as that has 6 ingredients. And that one is quite easy and fast to make.
Also, things like spices can increase the number of ingredients significantly without increasing the complexity at all. As a matter of fact, not all recipes in x-ingredient cookbooks have x ingredients on EYB, because they might leave out the "suppoting cast" when counting ingredients.