Part of it is growing up, but part of it is (obviously) that plays aren't meant to be read. There's a lot of detail and tone that doesn't really come across properly, and it sucks the life out of the story. Not sure why it's such a common practice in school.
I realize I'm probably wrong about my assessment, but I've tried to watch performances of his plays as an adult and the outcome is always the same: His words just sound like random noise to my brain. They bounce off.
And yet I love Cormac McCarthy. Blood Meridian is one of my favorite novels. Go figure.
The other books mentioned (Gatsby et al.) really require context, but literal pigs sitting down to dinner with powerbrokers is something a 14yo can grasp.
My teacher tried to talk me out of it, but I insisted and ultimately she let me do it because Billy Budd is very a short novella.
To this day, Moby Dick is still my second favorite book of all time (with my all time favourite being The Lord of the Rings). I only read it once, 20 years ago, for a book report but it really stuck with me.