why does Newton’s 2nd law only work with absolute accelerations?

Answer

May I rephrase your question to:

“Why does Newton’s second law only work in non-accelerated coordinate systems?” I assume that by absolute acceleration you mean the acceleration in a stationary or non-accelerated coordinate system.

Newton’s second law gives a proportionality between an applied force and the acceleration that results. The proportionality factor is the mass of the object. Since force and mass are quantities that do not depend on the coordinate system, the acceleration must also be the same in the different coordinate systems where Newton’s second law is valid. The same acceleration of an object is only measured in coordinate systems that move with a constant speed relative to each other.

You can clearly see that Newton’s second law no longer applies in an accelerated coordinate system. Suppose you are in a car traveling at a constant speed. At that moment no force is exerted on you and you experience no acceleration. You automatically take the car as a reference system. If the car suddenly brakes, you will experience an acceleration in relation to the car WITHOUT a force acting on you. F is clearly not equal to m times a.

Newton’s second law can be patched up again by introducing apparent forces, so that they can still be applied correctly in accelerated coordinate systems.