Because learning terraform or cloud formation takes time. I am an average developer and I understand foundational infra theory. I know I can click around AWS without help in order to deploy a CRUD app. Why bother with scripted infra if I am going to revisit it once a year to fix an issue?
If you're using AWS you probably should script it for nothing else than to help other employees / new hires understand what's going on. Something like Heroku is usually a better proposition if you don't want to script anything.
People writing/operating serious software never make config changes through the UI. AWS console then is mostly just to view/investigate what is already configured
