There are two kinds of people in this world: Those who think that writing a book, and even more so writing several books, will make you wealthy; and those who know better.
Yes, there are some exceptions to that rule - J.K. Rowling springs to mind, obviously, and a few others have had roaring success and become wealthy from writing. But taking that for the usual outcome would be like supposing everybody who dabbles in IT will end up like Bill Gates.
This is why I'm always delighted when real authors who have published several books write about their writing income honestly. Jim C. Hines has been doing this for several years now, and you can find his most recent post here. Kameron Hurley has very recently done something similar on her blog.
It's very interesting, and it can be quite sobering. I know, from my own experience, that you don't write a book for the money. You write it because you want to write it, and when it's published and you get some royalties from it, that's a reason for happiness, and getting some money is a good reason to find a publisher - but it will usually not turn out enough money to let you make a living, even if you are writing full-time.
That's a bit like knitting, actually. Most of us knit because they want to knit, or because they like the finished thing that they make themselves, or because there's no other way to get that special thing they have in mind... but if you just want socks or a sweater, it's usually cheaper to just buy them. (Never mind easier and quicker, too.) Most writers just like to write.
And without that, our world would be a little poorer.