There's no doubt that lap and shoulder belts are safer than lap belts alone in both cars and planes. The FAA already requires shoulder/lap belts in small general aviation craft, but not (or not yet) in large passenger aircraft. If you notice, the crew on those planes wear 4 point harnesses.
In any case, there are multiple challenges to implementing safer passenger restraints on commercial aircraft. First is just the engineering challenge of where to place the upper attachment point. In a car, the frame of the vehicle is close and easily accessible while in a plane it's not. That means adding the required structure to the seat.
Adding that structure to the seat will inevitably result in a seat that is more expensive, heavier and physically larger. Just replacing all of the seats on pasenger aircraft would be cost-prohibitive on its own, but a larger seat means there can be fewer seats/plane which drives the actual cost even higher. Finally, the seats would be heavier. Heavier seats mean greater fuel consumption also resulting in higher costs.
Finally, there's the behavioral aspect of shoulder belts. Passengers won't like them, especially on long flights. Flight attendants already have their hands full trying to get people to wear lap belts, a more restrictive belt will only aggravate that problem.