Fundraising and publishing are specialised skills that artists don't necessarily have. Also, lots of people, including many artists, like the stability of a steady pay check and employment rights, which means companies or co-ops.

The sci-fi author Charlie Stross has a series of blog posts on their site about all the work that goes into publishing a book and how much of it is done by employees of the publisher (and how that is not work that Stross can do / would like to do).