I'd say it's probably that they tend to be too easy, and I speak from my own experience, having been in the public service for more than two decades.
Generally speaking, targets are set so that they seem to be reasonably ambitious, but they are always deemed to be attainable, barring unforeseen events, so they generally tend to be not overly ambitious, which leads to a surprise when you see that close to half are either not met or have no target date for meeting them.