Believe me, if you're a founder, you want to exit as well. The problem is, the cost for being a public company has gone up dramatically, due to increased regulatory compliance costs.
Or is it that they don't want to, in which case the obvious question is why?
Sarbox 404 compliance costs hundreds of thousands per year on the low end and low millions in the typical case.
It adds basically nothing productive to a non-fraudulent company itself. (Maybe it makes you write down some policies and management controls and audit your compliance to them. Great. At what expense? At what RoI on that expense?)
"You're worth billions; you can afford to casually light a couple million on fire every year for an epic Sarbanes-Oxley auditor-led bonfire." That isn't obviously a good thing (or even a true statement) to me.
Another perspective is that there's a kind of members club investor thing going on, and the "actual market" (with their insistence on trivialities like profit and loss and audit of metrics) is an inconvenience to private games of high stakes poker that allow cap to be conjured out of powerpoint and closed door meetings. i.e, bulls%$t.
Personally I think AMZN proves that the market can evaluate growth focused companies that don't make money (compared to cap) just fine.
This amortizes the Sarbox waste across a larger base.
Your citing of Amazon (with a market cap of over $400BB) fits nicely into this model. They have done acquisitions and are amortizing their waste over that much larger base. Someone trying to go out standalone at $1BB market cap (which used to be viable) isn't necessarily viable just because $400BB is (more than) viable.
The reason they aren't going public is because they don't need to raise capital right now, and they know their financials suck and would get ripped apart in an IPO. They think "wall St doesn't get us!" They're waiting for their numbers to improve but it's not going to happen and meanwhile their valuations go up to unviable levels for the public market.
Obviously it couldn't have anything to do with ridiculous valuations that the companies couldn't possibly live up to on the public markets. It must be that pesky regulation. Someone should disrupt it!
(even your Wikipedia link says that the claims of correlation between Sarbanes-Oxley and reduced IPOs is questionable; maybe you should edit it?)