Because The Dark Knight was not about Harvey Dent and The Joker.How's that different from how "The Dark Knight" was really more about Harvey Dent and Joker instead of Batman?
(I'm not the biggest TDK fan but here goes

The arc is entirely Bruce Wayne's. And he ends with saying "My reputation as a hero is not what Gotham needs."
Ross has a "redemption" arc. He feels bad and he sits in a cell. (I have no idea what actual action he feels bad for. "I won't be as much of a jerk?")
They also try to spit the difference with the Thinker: Ross wouldn't let him go because Sterns was keeping him alive. Understandable. Motivating. (Still bad.) But ALSO Ross would never have become president without Sterns' strategy. Really? ("Step 1: Become Harrison Ford.") So he has the wrong but far more justifiable reasoning of "I will DIE". Isn't that enough? And also: Having a REALLY good campaign manager is supposed to be somehow discrediting? Now having a REALLY good CAPTIVE campaign manager is problematic. But it's not the captive part that is supposed to make Ross slimy.
Hey, who was Ross' VP?
The more I'm thinking about it the more wrong footed Sam's "when will I be enough" speech lands. It's not Sam. I mean, maybe this was some internal conflict he always had when he was hanging around with Super Soldiers and Gods. "Avenging is your world. And your world is... Crazy."
Maybe it's one of those Marvel things where they assume the audience has been with this character through thick and thin and knows the man and this is some kind of thing he was having with himself. But it's not. This is Sam looking at the audience and saying "I'M CAPTAIN AMERICA and when will that be OK?" Which is... What's a different more positive word for entitled? It's Sam saying that he is, well, WORTHY. And he is. But that still doesn't feel like the Sam we've been watching for 11 years.
You know, just like this movie needed Bruce Banner (if this was the story they wanted to tell) it also needed Steve Rogers. "No really, Sam. I picked you. For a reason. Believe that."
Sam doesn't have an arc. He stands still and the movie "catches up with him".