What kind of answer would satisfy your questions of why these theories are true anyway. I am not sure I can even imagine an ideal answer and this might be a clue that we are asking the completely wrong question.
I don't have answer on the "Why". I think that this question is not a scientific question because the answer could no be proved/invalidated by experiments. It's a philosophical question and some people find the answer in religion or philosophy, some just admit they don't know and others just don't care.