I can, have and do bake cakes without the use of cake mixes. It’s not hard, and I normally have most of the ingredients on hand. But cake mixes are very popular, come in a wide variety of flavors/types, and, generally speaking, are pretty good. They’re also cheap. And they store well, and efficiently on the shelf or in a cabinet. So it’s just super convenient to have one on hand. It also takes like 30 seconds to make. Add water, oil, egg, mix.