A cliché, yes, but also quite often true, no? So I've decided that we, as a board, shall attempt to explain the phenomenon. Or just list a load of bands who did their best stuff early in their careers and a load who didn't. Whatever.
My initial thought was that, maybe you just fall in love with the sound of a band when they're starting out, so when they change, it's difficult not to be disappointed. I don't think this is very often the case though. When the Pixies started out I was barely born; same for Guided by Voices. Modest Mouse and Les Savy Fav I wouldn't know existed until a good few years after they were founded. Same for most others.
So then I thought, maybe it's just part of the "band process". First you make that album you've always wanted to make. Then you maybe continue in the same vein for your second album. But then after that, you might want to experiment, a la Pavement, almost inevitably reducing your success rate. Or maybe you'll want to sell out, a la... well, loads of bands.
Or maybe it's just the fact that some bands spend 10 or more years conceiving their first album, but don't even have 2 to work on its successors. I don't know. Do you know? Tell me if you know.