Even top VCs need a portfolio of companies to produce a return. If you asked a VC to bet a whole fund on a single company :) and these are people who's full time job is to pick companies
Startup employees could pool their risk by creating their own shared investment fund which held all their shares/options in trust and spread out the winnings. The reward would be far less but more predictable. I doubt many would have the foresight to commit to something like this.
Start up employees take on far more risk than startup investors. Investors can diversify over 10s or 100s of companies but an employee has to pick just one and bet their career on it.