In a contemporary democracy we could, at least in theory, completely vote out literally every single pro-corporate, pro-war talking head in the next election. Now of course this won't ever happen, but at least in theory its an option. By contrast in communism, if one ends up unhappy with the system you have very little ability to change it.
Furthermore, in a capitalist system one can even start a communist sub-society. In fact there are many communes throughout the country, at least in the US. But in a communist system, you can't simply start a capitalist sub-society. The centralized nature of the system entails limits on freedoms, to ensure that every person is contributing to society as a whole. And this, in practice, trends towards dystopic authoritarianism in terms of how non-compliant individuals are treated.
The reason that people always claim that various efforts at communism weren't "real" communism is because the concept and theory of communism doesn't, and probably cannot, survive first contact with the interests and whims of humanity. By contrast I'd look at the overwhelming majority of the history of the US as an argument for capitalism. It's only relatively recent times, particularly after 1971 [1], that things have gone so terribly wrong.
