Credibility and providing liquidity for investors and shareholders are certainly important reasons why companies decide to go public. But there are also downsides to going public -- most notably the myopic time horizon of the public markets, which is a huge barrier to the kind of long-term product and user acquisition investments pursued by tech companies in particular.

You can weigh these factors against one another, but ultimately the fundamental reason a company IPOs is to raise a large sum of money at an attractive valuation. Given that investors are lining up to help private companies meet this goal, the ancillary drivers you mention just aren't enough to push companies over the edge into the public markets.

Seems like an IPO is riddled with all sorts of waiting periods and notifications and paperwork people will want to sue you over if they lose money. I'd definitely go the buyout route if the price was right.