As this "theory" of multiverses proposes (infinitely?) many parallel worlds which we cannot observe in any way, it's not a science at all. It's just another religion made up by people who want to avoid using that word.
Your assessment is overly harsh. First of all, 'multiverse' theory is often misconstrued and misunderstood. I think you are a victim of this. Second of all, while I agree it is not science, it is certainly not 'just another religion made up by people who want to avoid using that word'.
Sure, the 'many worlds' interpretation of quantum mechanics can sound kind of arbitrary I guess to someone who doesn't understand it, but it is just a sober, logical interpretation of a mathematical restatement of the evolution of the schrodinger equation of quantum mechanics (it is actually less arbitrary than the standard form of quantum mechanics, which is 'schrodinger equation' plus 'observation', which no one understands).