I think it's seen as a seperate art form now. Especially in the case of Gaga; she talks about her most recent few videos as short films. A good few other artists do that to, for example Michael Jackson.
Also, it's a golden opportunity for publicity/attention and often lyrics of songs aren't strong enough to portray this notably in a video. If the director were to take the lyrics and turn them into a visual story it would be boring. Bad reflection on song. So attention-grabbing visuals/topics/themes often have to be stronger than lyrical content. Getting an interesting video means a lot of publicity and popularity.