You are not entitled to the assumption that the other parts of the multiverse remain constant and uncorrelated to what you do. The multiverse could be superdeterministic. Failing to take into account your causes means you have a worldview in which there are two underdetermined events in the multiverse, the big bang and what you are about to do. Both versions can not be heeding local causation and everything is connected.
It makes life a whole lot more practical if you do assume it.
You are not entitled to the assumption that the other parts of the multiverse remain constant and uncorrelated to what you do. The multiverse could be superdeterministic. Failing to take into account your causes means you have a worldview in which there are two underdetermined events in the multiverse, the big bang and what you are about to do. Both versions can not be heeding local causation and everything is connected.
It makes life a whole lot more practical if you do assume it.