Because the income can change to zero in an instant due to several factors that are completely out of his control and that have zero recourse.

Thought 1: Wow, I've replaced my income

Thought 2: Or, I've doubled it.

It's easy to go for 2 for a while and then go with 1

or just wake up one day and decide to retire early

Early retirement (significantly early that is) does not tend to do well for folks who take it. The loss of mental faculties once you have a reason to stop using them is astounding. The ones who are most successful tend to "retire" and then switch to low paid or unpaid volunteer work full time.

It's easy. If I had a site generating that kind of money on the side for that little work, I wouldn't quit my job either. Why would you when you don't need to? Two income streams are better than one. I'd just pay off debt, then bank the profit and build up a nice cash cushion.

Assuming this would be his only income, the costs would breakdown to something like this:

$8,000 NET PROFIT

$5,600 AFTER TAXES (assuming 30% - largely depends on where he lives)

$4,800 AFTER HEALTHCARE (assuming $800/mo - w/family coverage this would likely be more like $1k+/mo)

So we're down to less than $5k take-home already, with a lot of additional risk. He would also lose other benefits like a 401(k), and other various perks. He said elsewhere in the Reddit thread that this "almost" matches his work pay, so I assume he is already making well into six figures, and already taking home more than this from his employer's paychecks (probably $6k+).

With that info, the choice is clear. Just keep the job as plan B, and enjoy a double income stream. Why take on significantly more risk for less money, with no benefits, when you don't have to?

These types of sites have a history of getting virtually killed in a Google change.

We don't know what his day job is. In that situation, I would quit a job at McDonald's, but I would not quit a job at a law firm.

Well he indicated his day job pays about the same as his sidejob currently does, so if he works at McDonald's he's not flipping burgers.

As someone who was using the Amazon Affiliate program, probably because it can go from $8-9k to zilch in a day. I wasn't making near that much, but our state was pulled from the program after a sales tax law.

I can think of lots of reasons. The main one is you should have a buffer to cover living costs before quitting for something that'll have variable income.

Because his day job is probably more stable.