>literature has improved in depth and characterization as much as any other field.

It hasn't.

1. Many fields experiencing development over the past century or two are much newer than literature, which might as well be a synonym for thought. During that time, those newer fields were, or currently are, in their early rapid-growth phase.

2. Literature is deeply on the subjective side of "fields", so it's a lot easier to argue that it has changed, rather than that it has improved.