In general, @Galastel is correct; the problem is the costs. That said, the first Harry Potter Book by J.K. Rowling contains "illustrations", my copy of "Harry Potter and the Sorcerer's Stone" contains graphical signatures, symbols or handwriting on 9 pages. These are all black and white only; and typically no more than a quarter page tall. In at least one case the text says the ink is green, but the image is rendered as black text. I notice also they appear without anything to the left or right; so no text has to be flowed around the image.
That makes them "low resolution" graphics that do not require special paper, ink, or typesetting. Some care must be taken in formatting the book to ensure there is vertical space to present the image where Rowling intended; that can tend to leave blank space at the end of one page (the widows and orphans typesetting issue). This is probably why the images are kept short (vertically).
I believe I have also seen low resolution full page black and white maps at the front of books, or at the front of chapters. Again, these would be not special paper, and not hard to fit into these positions (no typesetting issues).
When illustrations require special paper this creates a collating problem; the special pages must be inserted into the book in the correct positions before the book is bound. The paper is also less porous, and the cheap glue used to hold regular pages can release the illustration pages.
Automating the collation without endless paper jams demands printing the book in sections that are then stacked together for binding. This is prone to error. That is why in many illustrated non-fiction books for medicine or science, we find all the illustrations together in a block, instead of dispersed throughout the book near the text that references them.
And finally, publishers tend to be very discerning consumers of artwork, their business depends on extremely high quality artwork that sells the book. They will pay $thousands for illustrations. Compare that to the typical advance given to a first time author, in the $3000 range.
Author illustrations tend to be amateurish and poorly executed, publishers don't want to put their name to them, and paying for illustrations would make the book far more expensive than usual.
Things like standalone B&W maps, or signatures, handwriting or handwritten filigree like Rowling produced, publishers can work around that in layout. But for actual illustrations they want an experienced artist they trust.
I will also note that if your book relies on illustrations for any kind of clarity, your audiobook and visually impaired audience just left the building. That is also something for professional marketers to consider, the reduced sales potential of a book.