It's ridiculously hard to force predetermined lyrics into an unrelated melody. If you talk to any songwriter (other than rappers), the majority of them have the melody that they're going to use before the lyrics. Why? Because the melody and the lyrics need to match the composition. Every once and a while you'll see someone use juxtaposition to make a point. However, most of the time if the beat is happy, lyrics are happy. If the music's sad, the lyrics are sad. Let's not act like this is a new phenomenon. Now I'll agree that a lot of popular songwriters today are lazy and could be more artful with their execution. But the music almost always exists before the lyrics. That's usually how it's done and that ain't new. The only exception I can think of is rap, which has influenced songwriters to step away from melody and put more focus on punchlines or using too many words, i.e. Mariah's tinging. I actually wish more people wrote melodies first, because the adverse results in the monotone R&B/Pop that I hate.