Let me clarify: this doesn't apply to every self-published book.
You have examples like The Shack and Jesus Calling, which didn't start out as traditionally published books.
But often, in my inbox, I'll get queries for manuscripts where the author will mention they previously self-published the work (or sometimes they don't, and I find out on a simple Amazon search).
I often have to pass on these because most of the traditional publishers I have connections with won't take on previously published books (and yes, self-published falls under that umbrella).
Most often after I send that email, I get a response like this:
"It was just for friends and family. The thing only sold, like, 24 copies, so if they're worried about it being seen by too many customers, they don't have to."
Unfortunately, it's not as simple as that.
In fact, publishers want you to have astronomical sales on that self-published work if they'll consider taking it on. We're talking tens of thousands, or authors who have an enormous platform.
So why won't they take on the books that sold 24 copies?
Proof's in the publishing pudding in those sales numbers. The moment they see it didn't sell an enormous number of copies, they'll realize how much of the author's marketing efforts didn't appear to work on readers.
So, they'll assume if they pick up that book themselves that it won't have great sales, since it didn't sell well initially. The self-published book is a proof of concept, and if customers don't respond to it, neither will publishers.
A lesson I learned the hard way
At sixteen years old, I'd gotten tired of querying agents and publishers, so I decided to take publishing into my own hands and self publish.
Long story short, the book didn't sell well. I had almost no platform at the time, and managed to get one book signing, one school talk, and one publication to feature my story (a publication I was employed at).
Although I was younger, I discovered down the road I couldn't do much with the book since it only sold a few hundred copies. I enjoyed writing that world, but now can't do anything with the material in it.
Is there hope?
There are, of course, exceptions. Work on platform as much as you can, and sell as many copies of that book.
Some agents do take on previously self-published work. I wish I could be one of those, but simply lack the connections to get an author where they need to go on their publishing journey.