So the publisher says to the author "We would like to publish your book. An editor will be assigned to you."
Editor recommends changes. Usually they're going to be wonderful improvements. Usually if you don't agree, the editor will shrug, it's your book after all. But let's be hypothetical if you don't mind, OK? What if the editor wants changes you really really don't want to make? And instead of shrugging is very very insistent that you make them?
Can an editor tell the author of an 18th century adventure story between two boys living on the banks of the Mississippi that the book needs to be changed to a political tell-all nonfiction account of how Donald Trump's campaign was secretly funded by the Illuminati? Presumably not, but what are the limits of what is "reasonable"? (Please don't fight the hypothetical. I know this isn't likely to happen)
Let's assume also that the contract language doesn't specify that the author can cancel the contract and get the rights reverted back and look for some other publisher. What is to prevent the publisher from refusing to publish in any form other than with the unwanted edits, and refusing to kill the contract and revert the rights, so the author can't go find a different publisher?