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
It’s the kind of argument made by someone who only learns socialism from non-expert sources (breadtube etc) where theoretical precision is sacrificed for popular appeal. If you only ever listen to those people, and never read Marx directly, you are likely to end up with the most confused ideas.
This particular idea might have originated with Richard Wolff, who I’ve previously complained about so I’m not surprised if it did start with him.
Understanding capitalism in terms of workplace relationships differs from how others define capitalism. […] Using my definition instead, the last century’s epic struggle was not between capitalism and another economic system. That is because both capitalism and 20th century socialism relied upon basically the same employer-employee relationship for their workplaces’ organization. The difference was who the employers were: private persons in capitalism versus state officials in socialism.
This is why no one should listen to Wolff.