It's not. We have lots of socialism-like projects in the United States--from progressive tax rates to social security. All those projects do not "invariably" make a problem worse. All were labeled socialism at the time and still considered so by people today.
No society is purely laissez-faire capitalist, thankfully. It would be an ugly, ugly world.
But I agree that the larger scale, socialist projects mixed with totalitarian governments have been abject failures and produced as much misery as has ever been seen in human history. Those programs that act to cut off incentives from action to a large degree (which can be debated and should be tested) are the ones that probably don't work as well as others and I've come around to Hayek's notion of the importance of a price system. But a priori rejection of programs and efforts is anti-scientific.
As the argument proceeds, you'll go back and forth on what is defined as capitalism, what is defined as socialism, what socialist thought and classical economics gets wrong about inherent human nature, what Marx actually said, Hayek's retorts, etc. I used to read and study these things deeply and debated this topic ad naseum with philosophy, law, and economics profs. Those who start the debate with fixed ideas and most want to argue are the least likely to listen or think about the counter arguments or examples and others just don't care that much.
I'll also throw this out: people who think they've got human economic organization all figured out are fooling themselves and their hubris is palpable. Among other things, a good liberal arts education teaches you the history of ideas in the Western world and that many very smart, educated people in the past also thought they had everything figured out.
But thanks, snarl. I'll start using this thread to post various articles about socialism I come across.