[BVR]

You have a point there... AutoArt's hinges are pretty big indeed and unrealistic. Also, most bonnets and trunk covers on Kyoshos with those hinges you mean stay up.
It's actually a pretty nice system. The hinges Kyosho (and Minichamps) use slide into two slots instead of hinging in one point.

 

[BVR]

I think they use them because save space or something. It also just looks better in my opinion