The truth is that writers do not - repeat, not - earn a great deal of money. Yes, there are the few who make shedloads, but they're far and few between. It's like assuming that everyone who works in computers is as rich as Bill Gates.
Back in the real world, what do you get? When I got my first deal my agent told me (afterwards) she'd have accepted £10,000. This would have been £2,000 if it had been a literary rather than commercial novel. Not enough to give up the day job for in other words. Now, I got more than that, but that was then and times are harder now. Still, let's say you get £30,000 for your book. Yippee! You could live for a year on £30,000.
But before you break out the champagne consider this. First that's income, not a handout from the Lottery. You've got to pay tax and NI on it, after your agent has taken their 10 or 15%. That reduces it to about half. Can you live for a year on £15,000? Possibly, but now think about timing. When I sold my first book in October they wanted it to be a May launch, but it was too late for publication in May the following year. So it came out in May the year after that - 18 months later. Advances are stretched over the publication schedule - perhaps 1/3 on signing, 1/3 on hardback publication, 1/3 on paperback publication. Would £15,000 last you for 18 months?
You have no control over the publishing schedule. I was late delivering Another Woman's Husband (tut tut), so they allowed an 18 month gap for me to deliver A Single to Rome even though I produced it after a year. (It explains why Kissing Mr Wrong is coming out this May, 6 months after ASTR. I'm not working faster, just the output has caught up with the publishing schedule.)
The effect was, the advance for 2 books spread out over 3 years, again, 18 months per title. You have to be getting fairly large advances to cope with that. Yes, there are extra sources of income such as large print and audio rights, and foreign sales, but they're all unpredictable and if you're sensible you'll be tucking them away in savings accounts and pension plans. I did give up the day job, but quickly developed another in teaching creative writing and would strongly advise any writer to keep the day job going for as long as possible - or at least until the mortgage is paid off.