"Why was this made" is usually impossible to answer without asking the original decision maker(s). The only thing we can answer is "What effect does this decision have?"
As pointed out in the other thread linked in the comments, not using the full width has several effects:
- It limits visual clutter
- It preserves information hierarchy
- It preserves structural consistency
In the case of the Amazon front page, this is desirable: As users, we read the hero banner ("fill your basket with joy") first, followed by a casual trickle of categories we can buy from. Each item needs enough room to breathe, so that we can form thoughts of "oh yeah, I wanted to buy something like this!". A screen too densely packed with information would hinder such thoughts from forming.
That said: None of this really matters that much. Resolutions of effectively 1920×1080px (4k screens at 200% scaling are identical to this) and below still make up the vast majority of screens, and Amazon is handling those sizes gracefully.