An employee can get diversification in the sense that they can work for multiple companies. It's not at all unusual for someone in SV to get equity in 5 different companies in their career. E.g. one friend of mine first got equity in General Magic, but didn't really hit it big till Nest, decades later. (He did ultimately hit it quite big though.)

There is even a kind of natural balancing mechanism here, in the sense that the earlier you join a company, the more likely it will die quickly and free you to take another shot. So the riskier the equity you opt for, the more rolls of the dice you'll get.