Lack of proofreading has been the bane of writing in many locations over the last few years.
Do you remember back when newspapers came to your house and you paid to subscribe? Okay, maybe you don't, due to age or location, but it was a thing. Most people (at least among the college-educated folks I knew) subscribed to the daily local paper which was filled with articles, columns, and all sorts of written things.
Those newspapers had a little section, usually on or near the editorial page, that noted the mistakes from previous issues (rarely more than 1-2 days old). Factual errors (saying 14250 tons instead of 1405 tons) of course were top of the list. But they also noted regular mistakes. Misspellings of names and such.
As for ordinary typos, they were rare. I only sometimes found a single one in an entire issue. Ditto for magazines. And published books? hardly ever.
But now people get their news online, read only selected magazine articles sent to them in email or on the company's website, and download e-books for free or 99 cents. For some of these media, subscriber fees were just part of revenue. But now ads pay less as well. As recently as 5-10 years ago, a good blogger could get a decent income (not a living, but a good supplement) from hosting ads. Now the ads pay a fraction of what they used to.
All these means is there is less staff available to check manuscripts. It's endemic in online columns. Popular and profitable columns like Ask a Manager and Savage Love have 1, 2, or 3 typos and other obvious errors per column. These are not blogs and they have paid staff.
But you didn't ask about periodicals. You asked about novels. They are connected though. Less money in means fewer staff people. And the whole self-publishing trend has changed the industry. Whether it's blogs or novels, people do it themselves and they don't always think a professional editor is important.
How can a writer protect against this?
First, you have to care. You do. I do. But, frankly, a lot of people just don't. They don't notice the mistakes or they just don't think they matter. (And when they're in an environment where mistakes matter greatly, they are appalled and try to escape.)
If putting out a perfect manuscript is important to you, you'll budget the time and expense to make it happen.
Second, you have to oversee. If you have a traditional publisher, you have to check that they're doing what they should. If it's a top publisher, they probably are. But smaller presses might cut corners. If you are self-publishing, you have to make sure the work happens and then oversee it to make sure it's being done right.
Every novel needs several stages of editing. I can't list them all off but, if I were self-publishing, I'd find the lists online and in books and study them and create a version that worked for me.
What most self-publishers don't understand is that editors can't be the author and that they can't be amateurs. Sure, use beta readers and friends and family, but that's not for pre-publication editing, it's to get the manuscript to a stage where it's ready to submit to publishers. The editing comes after acceptance.
Pay for the work.
If you have professional standards for your work (and we all should!!), then hire professionals. An editor to work on structure and the big picture. Editors for scenes, dialogue, and smaller things. A proofreader. And someone to do the formatting setup for e-books and/or printing. One editor might be enough and some editors can also proofread. But the process takes multiple steps and can't be done all at once. Not for something large like a novel.
Third, do the final read yourself.
May all our works be perfection!