I think the deal is that the emotional beats in it work so well that the plot can be set aside. It's also not a problem that those beast aren't particularly clever or imaginative.

It's like a jump scare in a horror movie or Old Yeller. Even when you know what's going to happen you still startle or shed a tear. Honestly even when you know you're being manipulated and think it's cheesy you still get that response.

The weird thing to me about Shawshank is that it's not a monster out of the dark or a dog getting killed. It's not a cathartic thing but something else.

This is the great thing about art, there's no objective measurement, people are free to disagree and like what they like.