Because of other stupid laws that mandate high explosive air bags that can stop an adult without a seat belt that are able to kill even adults weighing under 120 pounds if they are unlucky. Thus we must keep all small humans out of the front seat because we aren't allowed to turn the airbags off.
I know; I find it amusing that I’m told to NOT put my 12 year old boy in the front seat as the airbag might kill him, but he is TALLER than me, the driver!
Guess the airbag would kill me too.