The theory, is that you have a certain percentage of the company (whether that is 10% of whatever), that is somehow "yours". But this is a private company, and you are a minority shareholder who presumably hasn't put much in the way of cash equity. What's to prevent the shareholders, after you leave, simply from dealing you out? There are all sorts of ways of doing this. The existing shareholders can get a liquidation preference in later rounds. The remaining co-founders (who presumably hold common), get paid out with a consulting bonus that acts as a drawdown on the equity, resulting in nothing for any of the minority shareholders after the preferred gets taken care of. If there are employees during a liquidation event, they can be taken care of with "Retention" or "Continuity" bonuses, and not rely on their holding of common.
Once you leave a company, I wonder how often (in the real world, versus they way we all want it to be) you simply end up with, completely legally, nada.
YC would likely know very well the answer to this - I bet it is north of 98% of the time.