When you're out of college, you're used to living on a shoestring budget. You can get by without quite little because you haven't really raised your standards of living around a big salary.
When you've worked for a few years, you've likely saved up a decent amount of cash (unless you live a life filled with consumption). You may have to cut back on all the perks but you can still get by.
Most of the people I know who want to start a company don't want to risk their current cash flow. Yet all of them have been working long enough where they've likely saved up enough money to go a long time without making money and be ok. I think the only thing is that people raise their standards of living (more expensive rent, big car payments, high rent/mortgage) such that they feel they must continue to make their current salary in order to "survive."