[pdq]

It's based on survival. If you have survived two years, there's a much better chance you will survive for the next two years, than the chance of two additional years of survival after only the first 6 months.

The longer you have survived, the more mature you likely have become. This means you go from a demo, to a prototype, to a working product, to having a pilot customer, to have paying customers.

It's certainly true that some companies get tons of money without really being a mature company (especially in these days). The investors backing them are really pushing for a moonshot, so they invest tons of money and expect to spend the money quickly. Those are cases of less mature companies basically playing the lottery, and I'd agree it's pretty risky.