Because we're clinging to the false hope that it will get better due to our 'investment mentality'. Because we've invested so many years in the series, we don't want to just give up on it like that.
With that said... Naruto was a very good show. Part 1 was amazing; The start of shippudden was also pretty good. The problem came after the pain arc... From then, the series started to fluctuate heavily and then started to decline rapidly into bullshit.
I'm one of those who still hope the manga can get better even though I've been 100% certain it would just become more and more shit since 100+ chapters ago. On the bright side, there are still some good chapters... well, not these past few of course.