1. It's easy to make songs too busy (i.e. in the melody or riff), which makes them overwhelming. If there's too much going on the audience won't know what to focus on and they might miss the sweet parts you're especially proud of. And writing a song with little fluff forces you to make the few parts you do have stand on their own (see The Strokes).
2. I think people generally care more about how lyrics sound than what they're about. I think that cool-sounding and meaningless is better than boring-sounding and insightful.
3. Not only does holding notes in the melody keep it simpler (see #1) but it also sounds sweet.