For example, if organizations price-fix, it heavily encourages a third party to undercut them. If they try to prevent the third party by legal means, then that's not capitalism.

If the goal is profit, then using any means available to increase profit is the promoted method. This includes creating barriers to enter into competition. This could be things like temporarily selling at a loss until your competition runs out of money. It could also be using your money to influence politics to get laws in place that make it harder for others to compete with you. It could also be many other methods.

It also means increasing profits through other means, such as cooperating with other companies to not compete (this is called a trust, and it's supposed to be illegal, but we all know it isn't always, for example the oil industry). If they all agree to not lower prices to compete with each other then they all make more money at the expense of the consumer. Obviously this is bad, which is why most capitalist countries are supposed to prevent this by law (so, obviously capitalism isn't that great alone), with limited results.

Capitalism also assumes perfectly rational actors in order to have good outcomes. Anyone who's interacted with another person knows this isn't possible. Without perfectly rational actors, the "best" outcomes are not guaranteed. There are far too many ways to obfuscate information and manipulate people. For example, in the case of a trust forming the consumer likely has no way to recognize that in order to work for their own interest over the interest of the companies trying to screw them over.

Basically, capitalism leading to ideal outcomes is a fairytale told by capitalists to ensure they aren't questioned. They tell you that it'd your fault if you don't get the best outcomes, but this isn't true. They know it isn't true, but it's in their favor. They use their influence to make sure the fairytale stays intact though. Capitalism is the newest large religion. It asks for faith, takes your money, and provides you with nothing.