“Why not have a loaded firearm on your lap and ready for action?”
Because it is not really “ready for action”. If you have to brake hard, make a sharp turn, take evasive action, it can easily end up on the floor and under the front seat.
It is a lesson from the FBI Miami shootout with Platt and Matix in the 1986. Several FBI agents put their firearms on their laps or on the car seat, then spent most of the gun fight looking for them after they bounced around the floor as they crash stopped Platt and Matix in their stolen Monte Carlo.
https://en.wikipedia.org/wiki/1986_FBI_Miami_shootout
I read an article about how most British police units do no carry guns so a special unit which did had to be called in. One day they will learn. Maybe.
Excellent. Thanks.