Herein lies the problem. Founders are uniquely well-placed to evaluate the risk, and uniquely psychologically motivated to evaluate optimistically.
Because of that, founders sell equity dear.
If the generous equity offer is reasonable, then would be reasonable to make two offers, one with only cash and one with generous equity, in an "I cut, you choose" scenario. If a company has taken funding and isn't willing to do this, then employees are being asked to take risks that the financial backers are not willing to take themselves.
[Edit: removed something about a typo]