Because it's a week lol you're talking about losing water from sweating, stored sugars in muscles from exercising, and a teensy bit of fat loss.

Yes, and that is visibly noticeable on many people.

When I switch from bulk to cut the cut starts to take effect like almost immediately, and I slim down significantly within a few days. I know it's mostly glycogen and water, but it physically looks very different after the water wooshes out of your body and your muscles become more visible.

(Also, it's not exactly sweat, it's that higher glycogen levels are bound to water molecules, which get released and can actually be used by the body or discarded as excess as the body seeks an equilibrium.)