Memes

Some things aren't compatible, and trying to combine them results in having one with the skin of the other. Liberal democracy, for instance, is incompatible with monarchy, and we can see in the UK for instance how the liberals kept on gaining ground until the monarchy became totally symbolic (a very expensive symbol, but a symbol nonetheless). If the workers having democratic control of the means of production precludes capitalists having plutocratic control, it's very simple. Put in the same environment, these two forces would bear what Marxists call "class antagonisms" towards each other and struggle until one was subordinated to the other.

The US (we'll use that example because I know it the best) does borrow ideas from various economic theories, those being various sects of classical liberalism, Keynesianism, and the odd bit of Austrian School or Christo-fascist policy. That is to say, different sects of capitalist ideology. You do not see policies based on Marxian economics or anything of the sort, it's just empirically not how US policy gets written, and that shouldn't surprise you.