Because I’ve talked to people who say things like that and it just makes me lose my faith in humanity a little 
But I have to remind myself that this tagline is some western Marxist (or maybe socdem?) verbal diarrhea and isn’t common among the people who matter
Exactly. Marx never really used the term capitalism as such, except as an adjective e.g. capitalist mode of production. The point wasn’t to define an abstraction but to explain a concrete, historical form of society, in terms of its particularity (the capitalist form) and how that is the manifestation of the general or universal (the mode of production) which is independent of the form of society.
Some early Marxists tried to generalize it as a theory of commodity society or of money but even that is imprecise, as Rubin explains in his essays, partly because commodities and money can and did exist outside of capitalism. Capitalism is about capital.