Cutscenes work a lot better (more immersive) if they can correctly reflect runtime-defined assets, e.g. your own character with your customizations, gear and clothes, etc, or the dynamic state of the environment in which gameplay was happening: destruction debris, current time of day, and such.
Cause they want to push the limits and make their engine look amazing. Also, if they research hard enough, in-game becomes nearly as good as video to the point you can’t tell.