Leaving means some wont see their equity grants vest. Some may be forced into major tax payments if they leave (due to options.) And some lose it alltogether. The way these things work, sometimes you end up paying enormous taxes and still end up with largely illiquid equity, which may remain illiquid for years. It is hard enough to land a job where the equity is worth something. People stick it out because a liquidity event can be life changing and liberating.
I realize equity agreements are created for retention, but they have the unfortunate impact of keeping people in jobs when it is better for both employee and employer to part ways. Something needs to be done about that. I don't know what.