Cranks are not just stupid people who think they are clever: cranks can be – and very often are – clever people. The important thing about cranks is that
cranks don't realise when they don't understand something: for cranks, there are no known unknowns, all unknowns are unknown.
The reason why this definition works is that when a crank approaches some field which they don't understand in some critical way then they fail to recognise that they don't understand what they are doing, and blunder on anyway, producing rubbish, but never understanding that what they are producing is rubbish.
Famously, the Dunning-Kruger effect implies that people who indeed are just not very good at something tend to be the ones who think they are best at it. But this isn't the same thing as saying that cranks are stupid: quite apart from anything else people suffering from the Dunning-Kruger effect are not necessarily cranks: being a crank takes more than that.
One group of people who very often become cranks are narcissists. This is pretty obvious: if you think you are, basically, some kind of god who is just inherently better than everyone else, what are the chances that you are going to listen to the advice of these lesser people?
But, once again, cranks are not stupid people. Cranks can be very smart.