The promotion of brainstorming as a starting point is all the more surprising as it is not the origin of most ideas: The things you are supposed to find in your head by brainstorming usually don’t have their origins in there. Rather, they come from the outside: through reading, having discussions and listening to others, through all the things that could have been accompanied and often even would have been improved by writing.
If you’re finding it hard to write, go do some reading or research and take notes.