Objectivity versus Subjectivity

I often wonder what makes writing for TV shows and movies different from writing stories or poems. Obviously of course there's something called a screenplay involved, but I'll be totally honest and admit I don't understand what that is exactly. You can often tell how good a show or movie's writing is depending on the quality of said show or movie and you can do the same with novels. Perhaps that's what connects them all. But then what about plays?
I often feel this is different because the quality of how the written words are carried out carries the show. Now that I think about it, it's the same for TV shows and movies. I guess, overall then, that you have to have a good writer behind almost any and every media form. Often, that's why I'm shocked when songs that I consider absolutely terrible become popular. But again there's no objectivity, only subjectivity there.
When I was in high school, I took a journalism class freshman year and it is the only media form I know of that, with the exception of opinion pieces, relies entirely on objectivity. It's often why I hate seeing a certain politician mock the media or step on them when they try to do their job. But I won't get any more political.
It's just amazing how much writing is used today, even if we don't realize it. I think I'll chew on this for a while.....