Why are theories and laws not absolute?

Because they may not apply in all situations.
The first law of thermodynamics is true everywhere.
The second law of thermodynamics isn't true inside the event horizon of a black hole
The third law of thermodynamics isn't a law but merely a point of reference to make measurements from.

Newtonian laws of motion are not absolutely true but are only approximately true but valid for things that travel at less than 1/2 the speed of light.