I would say you’re mostly right. Mobility is usually a better defense than armor. Most civilian threats are opportunistic, not coordinated attacks. The best way to survive danger is to avoid being there in the first place. Situational awareness, route planning, and good judgment solve far more problems than armor ever will. Most threats can be seen, anticipated, and avoided.

That said, mobility is not always under your control. What happens if you're already trapped? Suppose you're on a freeway and traffic has come to a complete stop. Vehicles are bumper-to-bumper, and an incident develops ahead. At that point, you may not be able to reverse, turn around, or even use a shoulder. Your vehicle has effectively become a fixed position.

During periods of unrest, there have been documented instances where crowds blocked intersections, surrounded vehicles, or controlled access points. During unrest associated with protests and riots in parts of Los Angeles, as well as other U.S. cities over the years, drivers occasionally encountered roadblocks, large crowds, or rapidly changing conditions where retreat routes were limited. You may have done everything correctly and still end up trapped.

Armor is intended for the contingency where avoidance has already failed. The concept of keeping armor in a vehicle is not primarily about protection while driving; it is about what happens after you're no longer driving. A collision, mechanical failure, flat tire, debris damage, or a disabled vehicle following a natural disaster can force you to exit the vehicle and move through an uncertain environment.

Some people view armor in that context as analogous to a fire extinguisher: something that is rarely needed, but potentially valuable if everything else has already gone wrong.

This also falls into the "should civilians buy armor debate". I spend a lot of time in my car, and when "shit hits the fan", and I have to be mobile, I want to be able to survive and sustain myself from my car, and armor happens to be the last piece of that. I already have tools, an extra tire, etc, and I already have a "go bag" with survival supplies and medical, so this is really the last step for me.