Valuations that in hindsight end up being too optimistic aren't unique to public markets. WeWork didn't have to go public to be valued at $47 billion. In fact it was the act of trying to go public that brought the valuation back to reality.
valuations of private companies are even more volatile than for "public" companies. in the case of WeWork it seems to me it was more like a scam of investors who fell for it. I stopped wondering when I read that some of the buildings were owned by the WeWork founders and leased by WeWork.