For much of the time, a lap belt is probably adequate - if you're at FL360 it'll keep you in your seat during turbulence and negative-g, and any force serious enough that a harness is better would probably mean that the seat belt is low on your list of problems (fortunately not many commercial aircraft fly inverted, and then not usually for long). When you come close enough to the ground then a full harness is much safer to keep you in your seat because the potential for a high-g impact is much greater. If you notice, the crew have full harness belts.
When you're sat in a hold for half an hour you don't really want to be restrained in your seat, it would get uncomfortable. Car seat belts, with the inertia reel, have improved a lot from the fixed ones we used to have, but the added weight of several hundred of those would be significant.