Because it normally gets to formulaic, cliche', generic, soulless, it loses focus on what it wants to be, random power-ups appear out of now-where, plot conviences and plot-holes, everywhere, everything becomes so dragged on, etc. The whole series becomes a mess, starting with a ton of potential, and I hate wasted potential. Just speaking from personal experience, of course. Espically with the "Shonen Jump" stuff. A lot of it is just plain stupid as well, like Dragon Ball Z.