Because they've had a long sleep? It's not that we're taller in the morning but that we're shorter in the evening; You get up in the morning, walk about all day; gravity, compaction; When you get to sleep the body reverts it's stresses..
Your intervertebral discs have a high water content. When you're upright, weight through the disc gradually squeezes the water out, and once the weight is taken off the disc at night it rehydrates.
MOst people dream about ebign taller while in deep sleep. Many studies have shown that while this is a factor in ones height, its more about confidence. A man must be confident in his manhood as well as his height and may SEEM but isnt actually...rather he is shorter at night.