The Laws of Dry/Solid Friction - such as that the friction force is independent of area of contact, proportional to the normal force between surfaces, and is independent of their relative speed - are empirical laws based on observation, like Hooke's Law and Ohm's Law. They are generally and approximately true in most situations for relatively small forces and speeds. Various theories are put forward to explain them, such as the making and breaking of bonds ('stick/slip'). But that is only one of several processes which are involved. Real surfaces also have small amounts of contaminants such as oil or grease which provide lubrication and complicate the situation.
Like all empirical laws they break down in more extreme situations or when tested with greater accuracy. As the speed of relative motion increases the friction force does in many cases decrease as you suggest it should, because of heating or aging.
We assume the Laws of Friction are true because they are useful, convenient approximations. They enable us to calculate and predict approximate analytical results without too much effort, especially when our object is to learn how to develop and use models to solve problems, rather than to model a real situation accurately. When accuracy does matter, more realistic empirical laws are used instead, and because of the complexity of these laws numerical calculations have to be made.
https://hal.archives-ouvertes.fr/ensl-00589509/document - courtesy of AnnaV