Because they are so foolish/over confident as to think "it won't happen to us"! That or they just don't value or love their children. It always irks me no end to see this terrifying practice. 

Like I've met some women who say their kids won't stay in the car seat unless they sit with them. And since its so "important to their marriage and love for their husbands" that they accompany them in front they just keep the child with them *shudders* Honestly where is their sense? Children don't belong to us..they are entrusted and a trial upon us. Making sure they are brought up well and remain unharmed is our responsibility and far more important than your spouse driving alone up front...sheeeeesh!