It's somewhat faulty to compare "commercial airliner" seatbelts with "car" seatbelts. A more accurate comparison would be to compare commercial airliner safety equipment with bus safety equipment as a baseline, then add the air specific items. Both count on the seatback (or empty space in front of the first row) as a part of the safety mechanism, hence no shoulder belts needed. Air travel adds the issue of turbulence strong enough to toss one into the ceiling, hence the lap belts (although, arguably, they should be required in buses too in the event of a rollover).
Now, if you compare car seatbelts with private single-engine airplanes (which are car sized and have similar cabin arrangement as a car), you'll find that they have the same lap and shoulder harness configuration as a road-going car. Newer private airplane seatbelts even have integrated airbags in the lap portion (mostly because it's easier to put it there than to try to find a space on an already completely full instrument panel/yoke).
The "seeming flimsy" part of your question strikes me as odd and overly subjective. I've never seen an airplane seatbelt of any kind that's flimsy compared to a car. They use the same materials, in the same widths. The buckles may be different, but that's mostly and age of design issue, and has nothing to do with strength (they all have to meet similar requirements).