Matt Levine's take is this is due to fact that "private markets are the new public markets"[0]. The lines between public and private markets are blurring so this is a prudent move by A16Z.

TLDR;

As companies stay private longer, and get bigger and raise more money while staying private:

* The secondary market for private shares becomes more important.

* VC's now may have more asymmetric information or more reasons to invest in public markets.

* Mutual funds are competing with VCs in later private rounds so why should VCs be able to compete with Mutual Funds in public markets.

* The obligatory crypto reference.

Another one he doesn't touch on is maybe it's difficult to efficiently deploy > $10 billion in just private markets?

[0] https://www.bloomberg.com/opinion/articles/2019-04-03/buying...