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5-point harnesses not bolted to a near horizontal surface (like a roll cage) can cause spinal compression. Fun.
And stretching out the length of the collision is the point of auto-tensioning belts and airbags, is as some_other_dave mentioned. They're supposed to allow movement with the idea of preventing internal brain injuries like the one that killed Dale Earnhardt.
That said, most of the weight comes from all the crumble zones and, in smaller cars, engineering them to be able to have a chance against the land ironclads so many other people choose to drive. Seat belts are a minor weight addition. Airbags wouldn't be too bad were it not for there being so damn many of them now. Cars are way too safe now, allowing people to do stupid things like text and drive with little worry of the consequence (yet, conversely, or maybe in awareness of their stupidity, they tend to buy cars based on perceptions of safety) that they'd be unlikely to do if the likely consequences were greater. I probably average seeing about a collision a day commuting to and from work, yet despite what looks like expensive damage (and I'm sure it is as even some idiot denting my Mazda3 in the parking lot cost $2500), everybody involved are usually standing around looking bored awaiting state police/tow truck (while raising insurance rates for the rest everyone).
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