Single player difficulty doesn't scale?

I play a lot with friends and go back to my single player where I’m further along.
When I get back to my game it’s worthless to play as the difficulty apparently doesn’t scale and at level 42 I get nothing useable. I breeze through it.
Is this something that may be fixed?

If You want “harder” game then for that, there is TVHM and Mayhem I, II, III.

I’m looking for level based rewards as I’m going through the campaign. As it stands it has no rewards playing it single player since everything is 15+ levels lower than my character.
I’ll play the other game types later.
Not scaling the single player campaign makes it suck actually.

It won’t scale on normal, only on TVH, everyone gets overlevelled in normal mode to the end.

It scales. Just turn Mayhem Mode. After beating campaign.