Because the game engine alone needs another years work on it to be ready.
Sometimes it's hard to tell even if bugs are game engine or quest related.
A couple more standout examples that occur to me.
There's a spot near where you pick up the space paining that when you're driving back into town you see multiple car sprites (I think for the distant cars) plastered over your screen for a few frames. This seems to occur 100% of the time.
Rarely Palm trees flop over like they're made of paper.
The mountains you can see from the first Aldecados camp sometimes flicker out of existence.