Because you can start with a side project

If you quit your job then start a business, you’re taking a risk. Most businesses lose money for some time before they start making money. Unless you have significant savings, it might not be long before you run out of money and you’re forced to close your business and find another job.

If you start a business before quitting your job, you’re taking less of risk. It’ll be hard, for sure, to find the time to work on your side project, but you’ll find out whether or not it’s viable without going all in. If you succeed in making it to profitability, you’ll be able to quit your job knowing exactly how much money your business can bring in.