and he could feel it itching

no, royalties are not a significant source of income for most authors. if you wrote something like Project Hail Mary, then absolutely, the royalties would be substantial! but that's like winning the lottery. Ryan Gosling is not starring in the movie adaption of most books.

when you sell a book to a publisher, you get what's called 'an advance against royalties'. it's usually split into multiple parts, so you might get one third for the initial sale, one third for the final revised manuscript, and one third on the publishing date.

an advance against royalties is the publisher saying "we believe you will eventually make X amount in royalties, so here it is ahead of time." and then you don't actually see any money from royalties until the advance is paid off. most books do not make enough in royalties to pay off the advance (I have two children's books and one fiction book, and have yet to see royalties for any of them).

advances are still great, but you need to make them stretch! for instance, last year my net income was about 30k Canadian (21k USD), which is fantastic when you consider that I mostly made it through writing. but I'm definitely not a millionaire. I can live off my writing (not trying to undersell that!) but I'll probably remain in a low income bracket for the foreseeable future.

this isn't me making a fuss about pirating (I've pirated way too many shows to run my mouth on that lmao), but there IS actually a financial incentive to having people buy your books!

buying books shows the publisher that they can make money off you, which means they're more likely to give you advances for future books. and advances are what authors live off (if they don't have a side job). that's why I always push pre-orders so hard! you have to be a racehorse worth betting on, or else they send you to the glue factory....

Pre-orders and Amazon/Goodreads reviews are used as a gauge by publishers to predict how many copies they are likely to sell and therefore how many copies they want to print. They want to sell most of what they print rather than print too many and end up with a bunch of copies no one wants that they'll have to pay to store or sell at a huge discount or recycle, which is what happens to unsold books. Most publishers have a limited amount of money and manpower to dedicate to any particular book, so if there's a book building up a lot of buzz, that'll give them reason to put their resources towards promoting that book to get a better return on their investment. If the publisher can't make money off a book or author they'll dump them.

Generally, every aspect of the publishing and book industries, including writers, agents, publishing employees, librarians, and booksellers are overworked and underpaid. Unless you're Stephen King or already famous, you don't make money off of books. We do it for sheer love of the game.