I don't think it is fair to compare health insurance with salary, they're completely different animals.
#1: You cannot plan for the unexpected
When you quit your job to start a business, you're planning to lose your existing salary. If you've planned well, your living expenses (food, rent, utilities, etc) are known, so you can plan for them.
You don't plan to get sick or hurt, and even if you did, it would be very difficult, if not impossible, to accurately estimate how much it will cost you to get better. You pay for health insurance to help you pay for the unexpected, so its value is inherently volatile.
#2: You cannot "go back" if the unexpected bites you
If you're looking to start a business, chances are you are highly skilled, personable, and/or ambitious. These are all traits that lend themselves to being abnormally hirable, so if things didn't work out the way you had hoped, you can likely get back into the workforce with a similar salary from when you left. And if that doesn't work out, you still have a better shot than most at a myriad of other job opportunities that at least pay you something worth working for.
If you get sick and you have insufficient health insurance, there are very few ways to get around it. If you were diagnosed with cancer a day after you quit your job, and your current individual provider was unwilling to provide all of the necessary treatment, you're out of luck. That's the ball game. You can't just back out of your startup venture and go back to work to fix the problem.
#3: You can die
Your salary can affect your quality of life, but with personal and societal safety nets, you're not going to die just because you're poor.
The U.S. has no safety net that ensures you will be provided with all of the care you need to not die even from curable or treatable conditions. Medicaid is about the closest thing to such a net, but it doesn't cover every person nor every thing. No personal safety net will help you here either (unless you know a few billionaires that have enough of an interest in you being alive).