This is why the myth of "market capitalization" is so persistent. Often what you have at an IPO is a conversion of preferred shares to common shares. So effectively the 8 classes of preferred shares do become the same as the common, and the "simple" market capitalization number becomes real.

However.

How many startups never IPO? 99%? 99.9%? In acquisition or liquidation scenarios, those common shares are often worth a lot less than preferred. And because these scenarios make up a large proportion of the probability distribution, it means the expected value of common shares is a lot lower.

Even unicorns that are far more likely to IPO can raise bridge rounds that cause major dilution for the common shares, rendering previous market capitalization numbers useless.