There's a fourth: they serve the needs of their customers over the course of years and decades. which, as someone in the market for an email client I can fall in love with and use for the rest of my life; is exactly what I want.
Why does every startup need to aim to take over the world overnight? Is it not feasible to first take external funding to grow quickly in early stages, and then transition to a more organic growth model once you reach a point of profitability and sustainability (and maybe also slowly buy back shares in the company to eventually become fully autonomous)?
Its preferable to work on a project for a few years instead of half/full decades.
Once you raise venture capital, you have to go big or go home, in the span of a decade. You don't have the "grow slow and stay private" option after raising a Series A.
That being said, this is just the current, common state in VC-backed companies. With the weak IPO market and hostile public environment that forces short-term quarterly thinking, we may see alternatives like formalized secondary markets. Perhaps companies like Uber will be able to avoid going public and still provide liquidity options for their early investors & employees.