Just to expand on this more, there are some reasons why this is. The biggest one is that even if you have all cap-table information available at the time of your offer (which you almost certainly don't), you don't have to be told when things happen (bank loans, bridge loans from investors, terms of new investments, new employee hires, expansion of the employee pool of stock, more shares issued to officers, new stock classes and more).
And that is just as an employee; you can't possibly be on top of all of this information without being a C-level employee, and if you're constantly asking for updates on this, you're probably not doing your regular job. Even so, you may not be privileged to some information (investors getting more shares issued if target revenue numbers aren't hit, for instance). And you definitely don't get a say in any decisions that affect this (unless you're a C-level employee, again).
When you are leaving the company, there are so many ways you can get screwed. Regardless of the 90 day exercise window, unless the company is on the very precipice of IPO-ing, you're never going to find out the terms of new investments / bank loans / (see above). And honestly, the easiest segment of people to screw over are the former-employees. The company can issue new classes of shares to current employees that render former employees' stock worth effectively $0. The company has nothing to lose; current employees are happy because they get a bigger slice of the pie, investors are happy because they didn't have to give up anything; the only people upset are the people who aren't around anymore.
