We can theorize about why static or dynamic coefficient of friction is more appropriate, and why. However, the bottom line on why we use the static coefficient is that it has been proven, experimentally, to be the correct one.
As a thought experiment, try this:
You have a car sitting in a flat road. What is the coefficient of friction, static or dynamic? The answer is "Static." Now, roll the car very, VERY slowly (like one inch per hour). What is the coefficient of friction? Still static. Now roll it just a little faster and ask the same question. Now faster.
How fast do you have to go before the answer is that the coefficient of friction is no longer, "static?"
The answer to that question, in my mind, is, "fast enough for something(s) in the chassis/strut/spring/shock/axle/wheel/tire/road system to excite the rubber/concrete interface enough that radial vibration modes of the rubber in the tire were strong enough to lift the wheel off the road and keep it off." ("vibro-planing" if I may be allowed to coin a word). The speed required to do this, on a concrete surface in good repair, is probably an order of magnitude beyond the capability of any street-legal conveyance.