I think that it is mostly because the recommendations change faster than laws can keep up. Also some of the evidence is weak. But pediatricians prefer to error on the safe side. Where some manufactures just keep making what sold well to the last generation.

I believe the recommendation is until 4'9". Which for some is age 12 but others is younger. Ultimately the important thing is that the seat belt is on their lab and shoulder instead of their stomach and neck.