Unhelpful

I was thinking about the words “uphill” and “downhill” yesterday.

Why is it that both of them imply a difficult time? You can’t say “Well, my day is pretty bad, but after the midway point, it’s all downhill” to indicate that the day gets better after the middle. It still retains a negative connotation.

You can have an “uphill battle.” Why not a “downhill battle”?