If you have the luxury of a safety net or in-demand skillset, then yes, getting laid off is not a risk. If you have less means, and the costs (both monetary and social/emotional) of needing to relocate to find another job turn into something much more daunting. In that kind of situation, loss of a job is a very negative event, much more than, say, a 10% decline of a wealthy person's portfolio would be.

re why work for an employer when you could take your own risk: generally speaking the odds of losing a job due to layoff are still less bad than the odds of a bootstrapped business failing, even if attempting to start a business was a realistic option, which it is not for many people.