Your listing is not exhaustive - startups can also be acquired for politics, for marketing purposes, whatever. There is a lot of meat space things going on in the upper echelons of the US tech industry.

Recent history shows that an idealized view only focusing on fiduciary duty does not capture the whole picture of business in the USA.

Rarely does one acquire dollars for the sake of having dollars. Dollars are power tokens, and the acquisition of them beyond a certain point is almost always accompanied by a motive.