For exactly that reason. The job of a copywriter is to find the 'essential truth' in an image that could as banal as a five year old playing with a washtub.
That essential truth has to be something that a large number of people can 'get' within two seconds on reading it and require no further context or explanation.
And it has to be patently non-obvious or cliched. It's not enough to say 'they grow up fast.' There are also tricks of the trade in the way he has structured the sentence that you're probably not even consciously aware of - such as the punchline of 'capture it' being left to the end of the sentence so you have to read the whole thing to understand it.
Or the simple, evocative 'The joy of being five', which brings to mind all sorts of associations and sets the context for the punchline. 'Imagine what it's like being a child' doesn't have the same impact, does it?
If that sounds simple - try doing the same with a random image from Google Images or something you've seen in the news. You'll find that it's not easy. Copywriting is an art like any other.