A traditional publisher would have most likely turned any random author down in the first place as it was the norm throughout the history of publishing with high initial costs and a few publishing houses functioning as gatekeepers.
Then there were the so called (vanity) self-publishing houses who more or less capitalized on the rejects, while asking for a quite significant amount of money first to print and rudimentary, if at all, edit whatever the author offered. Here the gatekeepeíng factor to the public was how much money you had to spare to get your books printed and listed.
Nowadays with digital publishing the initial production costs are so low, that suddenly copy editing and proofreading by a person other than the author becomes a significant part of the costs, which it wasn't before, since that cost was dwarfed by the actual printing costs and was normally considered a given.
With no money to spare for third-party editors/proofreaders/layout this job is outsourced to algorithms and mechanical Turks with questionable results.
I suggest the best way to improve quality and acceptance of self-published work, is to actually pay a professional third-party person to proofread and copy edit your book first in a joint effort with the author. Only after that you should submit your work to a digital publisher. If amazon then still rejects this, based on customer complaints than we have an actual problem at hand which is not just the absence of any kind of human copy editing.
---
Substitute 'damn' every time you're inclined to write 'very;' your editor will delete it and the writing will be just as it should be.
— Mark Twain
https://www.goodreads.com/search?q=editor&search_type=quotes