Not everyone is comfortable with the unknown of not having a job lined up. Even the most technically skilled developers may not pass all of their interviews and some companies, especially the bigger ones, can take a huge amount of time.

What you say is true to a degree; if you have plenty of money saved up then you can spend a lot of time unemployed but this ultimately looks poorly on your resume at some companies and most people live closer to paycheck to paycheck than anything else.

I would never leave a job before finding a new one. Too much uncertainty especially in today's rough hiring market. You'd want at least 6 months of living expenses saved up to burn while you job search, probably more, and most people don't have that.