Because a band has to gel to last for a decent length of time. Bands who stick together for long periods of time usually have an aim or a specific concept, sound, theme, whatever.
Splitting is not always the end though. Take ABC for example (previously known as Vice-Versa). Band leader Martin Fry realised that after the first album, the second album would be compared to it. ABC then spent their career doing basically a different style every record. By the second record, there was only 2 members left out of the 5 or 6 they had started out with. Eventually, it was just Martin Fry as ABC. The original drummer has rejoined though.
Same kinda deal with The Human League.
I believe that music is to transport you to another world inside of your head. Whether this is done through suggestion, taking me into your mind or through charting the course of history, I always feel that the best bands make you feel like you are somewhere else.
Because of this, I believe that, objectivley speaking, the best band in the world might very well be The Legendary Pink Dots. Subjectivley, however, it's Swans and if you even think about disagreeing, I will kill you.
