To add to your last paragraph: The median income for
full time writers in the UK is below minimum wage. Most full time writers do it as a supplementary income only (the median
household income for full time writers is above the average), and the vast majority of writers are not full time.
If you as a writer use a paid editor and cover designers, odds are it'll take you several years to break even today.
Unless you're "discovered" by a traditional publisher and they think they have the next Harry Potter on their hands, even being traditionally published means next to no sales for a large majority.
So it's already about fun and fulfillment for most authors. I went into writing (so far two) novels knowing the above, and did it anyway. That said, some do go into it thinking they'll make lots of money, or desperately looking for another income source.