Question

Why can't energy be created nor destroyed and only be conserved or transformed?

Answer 1

Conservation of energy. ... In physics, the law of conservation of energystates that the total energy of an isolated system remains constant — it is said to be conserved over time. In other words, this law means thatenergy can neither be created nor destroyed; rather, it can only betransformed from one form to another.
It is much more appropriate to call it a law than a theory. A theory is an explanation, whereas a law is based on repeated observation and/or experimentation. If you are concerned that it isn't or can't be proven then you could call it a hypothesis, assumption, or postulate of conservation of energy. Of those three, postulate would probably be the best answer, as conservation of energy is accepted. However, the conservation of energy has been demonstrated so many times in so many ways, and in so many fields, it probably has more experimental support than many of our other scientific laws.