The problem isn't just in startups with stock options; another big place it arises is closely held businesses. You receive the family business as an inheritance and suddenly you need to pay - in cash - 40% of the value of the business. Such a large cash hit can and does destroy many companies.
The solution is of course to require the IRS to allow payment in-kind (shares). Then you could exercise your options and hand over ~20% of your shares to the IRS. Or you could inherit the family business and give the IRS 40% of the the company (which suffers no cash flow hit and continues normal operations). This additionally would prevent the IRS from overvaluing in-kind earnings.