Bleeding and brake fluid change instructions and seen many videos

I have read many brake bleeding and brake fluid change instructions and seen many videos. I am curious why none of them mention keeping the caliper compressed on each wheel in turn when doing a brake fluid change. Without keeping it compressed most of the old fluid will stay in the caliper which is exactly where you don't want old, burned, moisture laden fluid. Seems like a logical thing to do but there must be a reason why it is not mentioned. Is it because it would take more time?