This is not true. These systems are in fact flawless, but they include people.
People are severely flawed.
One of our failings is a tendency to project our common internal flaws onto external objects. Most would agree that the supposed defect of capitalism is greed, but greed is a human flaw. No mechanism is capable of greed. Actually, capitalism discourages true greed. A truly greedy person lacks the respect for money and property that a successful capitalist learns.