I believe generation comes in two types: inspired creation and uninspired creation.
Uninspired creation is when you force yourself to think of something, regardless of your state of mind. You simply squeeze something out, that most likely won't be good. But you're still forcing the pathways in your brain to generate it that you will train to be better at generating ideas. You're also learning your brain to generate a revenue making idea from a random thought. For me it helps to write these down to keep the creative muscle strong.
Inspired creation is when an idea strikes you and you just need to figure out the details. Kind of like an eureka moment. For me it feels like the more trained you are at uninspired creation the more inspired creation happens. It also helps to carve out the idea better. E.g. you have some thought of creating value but are uncertain how to extract value from that.
This is for me the practice. If we look at idea generation in general I think the answer is; because nobody has thought of it! Creativity and innovation are scarce resources. Having the guts to turn creativity into reality are also scarce. And last of, having the perseverance to sticking with an idea till it works is also scarce. So the reason it hasn't been done summarizes to; people are uncreative, afraid and lazy.