Because when companies dilute stock they typically execute it so that investors don't lose any money. Their percentage stake in the company goes down, but since the company is raising money at a higher valuation it balances out.
This trick doesn't work when dilution results from someone calling options but that's pretty standard and usually doesn't have much impact.
So it sounds like the only insurance plan against being diluted to zero is "I hope they don't fuck me" -- which sounds like a really, really bad insurance plan. I still remain unconvinced that they should be valued above zero.
The execution that Spivak described that causes the investors to not lose money also causes the employees to not lose money. If the value of a company goes up (because it is successful) your share price will continue to rise even if your shares get diluted due to additional financing.
