Why, because it contains the hook. Every song has approximately three minutes to tell a story, and each line has to be precise and to the point. So, time and space limitations combined with the fact that the lyrics have to be set to music are precisely why most writers use the tried and true formula. There are three popular rhyming patterns. The first one is the easiest.
Every line rhymes with itself.
This kind of rhyming pattern is boring and predictable, but writers still use it. The third pattern rhymes the second and fourth lines. Listen to your favorite CD and see if you can identify some of these them. The next consideration when writing lyrics is the number of syllables you use. Each verse should have the same amount of syllables.