Why write fanfic?
Because not every human activity has to be monetized.
Because sometimes I want to use a setting that's already established.
Because humans like to tell stories for the sake of telling stories.
Because the act of writing fanfic is very different from the act of writing fiction intended for the commercial market. For example: I don't have an editor. I can write whatever I want. I am unconcerned about whether the fanfic would be considered 'good' or 'bad' in the eyes of others. If people want to read it, then that's on them. Does writing fanfic make me a better (read: more marketable) writer? Probably not. Do I care? No.
Because we live brief lives in a universe that is largely incomprehensible and we all process information differently and do all sorts of things for fun. For example, right now, there's probably someone out there who masturbates by sticking a toothbrush into their peehole. This is no more or less weird than the time I spend writing fanfic, and at least fanfic leaves my urethra undamaged.
