Why are theories and laws not absolute?

Answer 1

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.

Answer 2

Theories aren't considered absolute truths because science is always changing; as new evidence is uncovered, a theory may be revised or replaced by a more useful explanation.