Well, that just isn't true. People who say that are using an excuse to be lazy and not practice. One reason is that adults don't use their imagination as much or as vividly as children. That doesn't mean we can't use our imaginations well, it just means that most people don't. There's something magical about the imagination.
Children know that intuitively.
It's a matter of focus. Children use their imagination to unfocus from their daily lives. They know how to daydream and pretend. This exercise isn't just for now, it's an all-day exercise. All day today, at every available opportunity, I want you to exercise your playful imagination.