If a company really believes that, they're free to back-load their grants and vesting. The reason they generally don't do this is because "dead equity" isn't a real problem. I'm nearly certain most employee options go unexercised. For one thing, most employees don't vest their full grant before they move on. And those that do often can't afford to buy it anyway.

The one exception might be the handful of employees that joined before the first big round, who might still be able to exercise at a negligible strike price. Again, I don't think it's a real problem in the grand scheme of things.