A business is exceptionally high-risk, if you value your time at more than zero. It's not like a savings account, where you put your money in, ignore it, and you're guaranteed to get it back someday. It's not even like a stock or a bond, where you have a framework to evaluate your investment, and can make prudent choices with a little bit of effort. When you start a business, you're investing your time -- your future earning potential -- into a vehicle that has a poor history of return, and no framework for evaluating the odds. That's as risky as it gets.
This point plays into your second comment: sure, it makes sense to get as much of your funding from outside investors as you can get on favorable terms, but it's naive to think that outside investors are going to save you from investing in your own company. Investors don't want to take the risk unless you have some skin in the game, and founders don't want to relinquish control before they need to do so. Thus, in reality, founders end up investing in their companies heavily. They're already committing their most valuable possession (time); it makes sense to double-down with money, too.