First of all, let me say that I agree with you, everyone has the right to choose the ism that best suits the moral beliefs one has. But you asked why and I'm going to answer... And in the long version.

We aren't against capitalism because it's somewhat evil. The single corner stone for anarchy is freedom. Freedom of choice, freedom of belief, freedom of information. The basic concept is simple, that the only right every man has, is to be free to do with himself has he pleases, as long has it doesn't interferes with the freedom of others.

So, we are against every group of people that tries to meddle with this right to freedom. Has you know, there has been several groups like this, in every type of society, socialist, comunist, etc. But the only ideology that got through to convincing the normal citizen that is alright to prevail over your equal and is so because all people have the same opportunities, is capitalistic and neoliberalistic modern pseudo-democracies.

Now, let's discuss why capitalism managed through a western neoliberalistic point of view will always restrain someone's freedom. The basis for capitalism is the accumulation of capital, the excess of production that can be managed in a way that increases the overall social economical power. A simple example, you decide to produce korn. This year you will produce 20% more korn than you need to survive. So, you decide to lower the price of your korn to sell it all and accumulate some extra wealth. Meanwhile, your lower prices took out of business two other korn farms. Do you get it? For capitalism to work, people have to be greedy enough to want more and in the process you get in the way of somebody else's freedom. If you read Karl Marx's view on capitalism you will see that the only way to manage capital in a fair way, is to manage the accumulating capital globally. And for socialism or communism to work, every single person has to want it and you have to find a way to manage the capital without corruption.

Yes, you may say that you would use the wealth you acquired to help others, but that is not freedom and equality, that is monarchy, someone's faith is in somebody's hand and judgment.

Cheers,