Because life and its aspects can be extremely difficult to understand without experience, to present a full level of complexity to a student or child all at once can be overwhelming. Hence elementary explanations tend to be simple, concise, or simply "wrong" - but in a way that attempts to make the lesson more understandable.
From wikipedia
An example in skating might be representing "setdown, glide, push" as three distinct parts of the stroke. It helps to introduce them that way, but it doesn't mean that's what actually happens - it's a tricky matter of timing and the likelihood is that a good skater will be pushing (perhaps at a very narrow angle and not pushing much) practically as soon as he's set down and there's no intervening "skating straight ahead on one leg" stage at all. But still, if someone's not reached this stage and is skating with short choppy strides, then telling them to extend their glide may well have the desired effect of slowing their cadence.
The argument for lies-to-children is purely pragmatic: we do it because it works. That doesn't imbue them with any kind of extra moral value, though: in a situation where they don't serve a pedagogic value, they simply become "lies".
