Why do leading ASICs manufacturers sell their chips?
I think the simple answer is that a company doesn't have to be a "Bitcoin believer" in order to manufacture Bitcoin mining hardware.  The companies that produce ASIC chips also produce other products for other industries.  They are perfectly profitable just selling their chips to mining equipment manufactures, and probably don't feel the need to enter the world of speculation on a young, digital currency.

At their core, these companies are silicon chip factories, and there is lots of demand for custom silicon chips in the market.  There's no need for them to change up their business model and become Bitcoin mining facilities.

Don't forget that during the California Gold Rush, the wealthiest man in town was the guy selling shovels :)