First, there are the actor s ego. They don t want to play a character who would say or do a thing such as you have written, but otherwise love the story and character, of course. Or else they fancy themselves a better writer than the writer. This, of course, can be true, since there are some bad writers getting paid to write scripts. But there are far more actors who have no clue about writing who believe otherwise.
Accents fall differently.
One liners come off as flat. Sentences go on too long or are too formal and rigid. Then there is the ego of the director, who s ideas don t necessarily improve your script, but simply make it his or hers. Maybe this director loves your idea, but has in mind a different execution. This is a matter of taste and is the unfortunate side of handing over creative control.