- Scaling down a company is generally hard and (lately) not done as often. It's pretty difficult to spend 6 years where hiring is your biggest priority (from executive level on down) and then all of a sudden it isn't.

- It's quite common for investors to push hiring more people as a way to get more stuff done. The general refrain you will hear from many investors on how to (move faster|fix problem) is to hire for it.

- The default thing to do with capital to invest is to acquire/hire.

I guess to summarize, staying lean just isn't a priority for these companies or their investors. The expectation is enormous growth or failure. The reality is often quite more nuanced.