Horrible direction

I tend to agree that the character work could, and probably should have been stronger and clearer, but I think they made a conscious decision not to engage us further in the mains, and I believe I understand the reasoning behind this.

The driving theme of this movie is vulnerability.
Even from the scene with Alex making stupid handmovements across the steeringwheel at high speed in order to be funny, we are introduced to a subtly dysfunctional couple, where he, being the idiot that he is, is trying to impress her, in order to score and win her over. He makes a long number of illconceived decisions with that motivation, being the idiot that he is. She, on her part, is trying to play along. She wants their game of seduction to work, but for obvious reasons, it doesn't. His idiocy is just too obvious. So much for the social dynamic.
At the dramturgic level, this allows the audience to have that sense of foreboding, and to know that this trip is a disaster waiting to happen to the two hapless victims of this awkward social game of attempted seduction they are playing. We slowly begin to sympathise with the woman, only as she begins to put her foot down and come to terms with the completely failed social dynamic that is their apparent relationship.
But mostly: this story is less about the two mains, and more about being exposed to a predator in the wild. They are presented in such a way that we know that they could both easily die, and that the narrative isn't a great hindrance, but rather a vehicle for their eventual doom.

In short, I agree that it could have been a more gripping movie, if the mains had been easier to sympathise with, and consider heroes. That just wasn't where this movie wanted to go.

To expand a bit on what I'd consider this movies aim: It wasn't an action adventure, or anything close to it. It was much closer to a classic slasher movie. You (or your moralising superego) sort of want to see the idiotic youths of a classic slasher flic get butchered and die screaming, because they are portrayed as asking for it. At the same time, you are horrified by the horribly gory and painful deaths they are exposed to. That's the whole idea: a cautionary tale, powerful enough to keep you entertained, and sensible enough to deliver a meaningful morale, to make sense of all the horror.