A single world admits many hypothetical situations, abstractions that express the features you care about. Knowledge of the world lets you judge their weight, decisions happen at the level of these abstractions, way above low-level description. So it doesn’t matter if the alternatives you are working with also in some sense correspond to things that are physically real. Even in a “many worlds” world, you’d be normally acting at a much coarser level.
A single world admits many hypothetical situations, abstractions that express the features you care about. Knowledge of the world lets you judge their weight, decisions happen at the level of these abstractions, way above low-level description. So it doesn’t matter if the alternatives you are working with also in some sense correspond to things that are physically real. Even in a “many worlds” world, you’d be normally acting at a much coarser level.