For one thing, bookshops will refuse to stock them. This is not because bookshops are mean and cynical; it’s because there are insane numbers of self-published books out there, and many of them are self-published because authors were either not good enough or not marketable enough for major publishers.
If you owned a bookshop (one of the ones that hasn’t already financially collapsed), wouldn’t you want to pick the best written, best edited, most marketable books?
On the other hand, here is an article about some independent books that mostly did quite well. This presumably involved a LOT of promotion work over time by the authors.
For those who want comparison numbers, the average book published by a large Australian company gets an advance of $3000-$5000 for children or young adult books, or $5000-$10,000 for other novels. Unless the book is a success (sadly, that just doesn’t happen often – a “success” would be selling over a thousand books), this is the only money the author gets.
But it’s almost guaranteed to be more than a self-published or POD author gets. So be wise with your work.