Can't the CEO be the product guy?

Yes. If you're good at product you're typically good at sales. If you're good at sales you may not necessarily be good at product.

Sales is about navigating conversations to match someone's problem to a solution. Product is about navigating conversations to understand problems and figure out what the solution should be in the first place.

Product CEO only works for companies where the product sells itself, which in my experience is only a small fraction of companies

If the product doesn’t sell itself, then the founders failed to make something people want. If sales lead the company instead of product, then market-making becomes the company’s actual product.

You are getting confused by product led growth. It's a popular tech business model but most enterprise software doesn't work that way.

That's frequently true, but it really depends. Some CEOs are the HR person, entirely focused on filling roles with people who know how to sell/build/whatever.

I do know one CEO who is "the engineer". They're in a bit of a weird market, and it seems to be working. I rather expect he'll move to a CTO role once they're actually in production, though, when the focus becomes growth.