It's because people are too focused on who controls the capital and not focused enough on what the capital itself is actually doing.

It doesn't much matter whether it's controlled by a Capitalist or a Communist if the person controlling the capital is a fucking idiot. Hell, it honestly doesn't even matter that much if they're smart, because the actual driver of growth has always been competition, which is only very indirectly connected to who controls the Capital, largely because it's pretty much always been taken at the point of a gun for all of human history and likely will be for as long as we exist unless we somehow manage to decide on post-scarcity society rather than infinite growth society.