Blog:Why many movies has to be different from the original source ?

In the Original Shrek! book it shows that Shrek is a villain and he has laser eyes as well with the Magic Word "Apple Strudel" While in the Shrek movie it shows that Shrek has to save Fiona and defeat Lord Farquaad (Fairytale Creatures like Pinocchio,Gingy,etc and Lord Farquaad are actually not from the original book anyway),In the Original Big Hero 6 Comic it shows that the whole setting of this franchise takes place in Tokyo with many characters in the comic are different than the Movie have,In Frozen Elsa is a Good Queen while in the original Hans Christian Andersen book Elsa is an Evil Queen,this also in Who Framed Roger Rabbit in which includes almost every crossover Characters appear in the movie as well with few personality changes to the Original characters that was appeared in the Original book Who Censored Roger Rabbit(Crossover Characters are actually non-canon to the Roger Rabbit Franchise anyway but oh well), If these were true Why Most movies has to be different than the source material have ?