If you really think the US went all in on Afghanistan with a world war level effort to conquer the country then I don't know what to tell you. After the initial invasion period the country really was a sideshow and became increasingly less prioritized over the years. By the end of the occupation US forces had shrunk to a minuscule size compared to what an invading army would look like.

That doesn't mean that the war was handled well, it clearly wasn't. But looking at a small force and confusing that with the entire might of the US military is baffling to me. "Losing interest" is a perfectly reasonable description of what happened.