Unchecked capitalism allows for the snowballing consolidation of wealth and power, and is therefore inherently authoritarian/exploitative of others. It is possible to have markets that encourage and reward creativity, novelty, and competition without allowing any person or entity to become too powerful (mind you, wealth and power are always the same thing - or more specifically - two sides of a single coin).
This is also why you will see leftists attacking liberals - because liberalism is simply an extension of capitalism: the allowance and encouragement of inequitable consolidation of wealth and power. An egalitarian ends must actively oppose, prevent, and even correct such consolidation. Capitalism and liberalism are the enemies of freedom, liberty, and justice.