The ones that do - and usually the few in this age group that succeed - are already independently wealthy from a combination of having a highly successful career and making good investments.
People that have $1M+ to fall back on - and can easily land another job making $400k+ - if things don't work out.
You don't see too many 35 year-olds with 3 kids and no savings starting Ubers.
Every once in a while you find an Elon Musk who already had plenty of money, and decided to start another business.
People with insignificant amounts of savings starting successful companies are almost exclusively in their teens or 20s.