Not true in all cases, but there are a few reasons that overclocking can be reasonably safe:
1) The manufacturer clocks are typically the highest that they can guarantee for a large batch of CPUs, meaning that a significant number can go higher without any issues.
2) Overclockers typically have better CPU cooling than the default (which the manufacturer clocks are set for), meaning that errors are less likely at higher clock speeds.
3) The same silicon is occasionally used for both low and high speed CPUs, just with a multiplier/FSB change. It is cheaper to manufacture than having separate designs. In this situation, it is usually safe to overclock the lower CPU to match or exceed the better CPU.
4) Most overclockers test the speeds they attain. If the CPU can manage 100% utilisation for e.g. 24 hours, then it is unlikely to have errors in the future.