The issue is liquidity. The tax comes due as soon as you leave, but you might not be able to liquidate your shares quickly enough to be able to pay those taxes. Depending on how your company gets valued these can be eye-watering numbers.

This is something you can solve with enough time, but if I get a job offer where I'm supposed to start in 2 months? Very inconvenient. (There are some ways to spread this out have this tax burden spread out over time, but it still represents significant friction)