Of course, but if your stock price drops by 25% or so then the RSUs still maintain most of their value. If you were issued $400,000 of RSUs and they're now worth $300,000 - that's still a powerful incentive not to leave before they vest.

On the other hand, if you're issued stock options with a strike price of 10% or so below the market price when they were issued, a 25% drop means that they're completely worthless.