Writerminded, I don' think it's (just) free, self-published or independently published ebooks that the OP is talking about.
Medievalist has said several times that major publishers used broken processes to convert their books to ebook formats, such as using scanning/OCR (yes, you read that correctly - they do that even though they own the original electronic copy), and such errors creep in.
On the other hand, I've just read a hardback copy of a currently popular book by an imprint of a major publisher on the future of the world and how wonderful it's going to be. I enjoyed it well enough, except that it has an "abundance" of typos and other errors. And sentence fragments (irony intentional).
This book isn't as bad as the OP describes ebooks, but this is a person with money and influence, and I would have thought the editing at the major publishing house would be better. If I had received the "final copy" of this book before printing I think I would have paid a professional editor or two out of my pocket to go over it, and gone over it again myself.
Printed books as a popular item are on the verge of going the way of LPs (which are still made, but sell
in the millions instead of
hundreds of millions). There's a small "resurgence" in LPs as people claim they "sound better." What they actually hear on old LPs is a greater dynamic range, an actual variation in volume between loud and soft parts, whereas modern pop recordings are "hypercompressed" and have the same "loud" volume all the way through. It's not the format so much as it is the processing.
I wonder that someday people will "rediscover" old books, that they have far fewer typographical mistakes than the commercially produced ebooks from the major publishers, and there will be a small resurgence in printed book sales (as if future-printed books would be more carefully edited).