1) The oft said "the movie is so bad it's good."
2) Sometimes you have invested too much time and mental equity to give up on the film.
3) The movie skims the surface of badness and you keep trying to convince yourself it will get better.
4) It's cathartic to yell at the screen and think to yourself "I could make a better movie than this crap."
5) You simply have nothing else to do OR you are doing something else at the time.
6) Ultimately, good or bad, if you are like me... You love movies. Why, oh why, must these filmmakers kick us in the dick so often?