IMO, the tailgate is sloped for a couple reasons. To help hold cargo in when it's down and if it is down and you back into something it should try to close instead of breaking.
Anyway a easy fix if you don't like it is to get some flat steel and a couple bolts and extend the bracket. I had a customer machine in so I did this quick with what was just laying around, just happened to be an old brake pad that was close enough. It should give you the idea on what to do though if you need/want the tailgate to sit flat and don't want to find the perfect length of cables. Tailgate will still function as normal also.
NOTE: This machine was returned back to normal after the picture was taken. The brake pad was just to show what was needed to be done with the flat steel.