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There are a few reasons. Their suits are pressurized. The joints of these suits want to fully extend due to this pressure. This makes it difficult to move and do things, and reduces precision substantially. Not only this, but the suits are quite heavy, and have inertia to slow movements. Lastly, the astronauts do not have anything to stand on. Fast movements will make their bodies start to rotate and further reduce the precision of their motions.
It’s precisely because there is nothing to slow them down. They move deliberately because if you flip something it’s not going to be slowed down by air. Move your arm too fast, and you’ll rap your wrist on something. “Every action has an equal an opposite reaction” said Newton, and without gravity to stick you to the ground that reaction has to come from your other hand or some Velcro gizmo.
Your actions on earth are made safe by gravity keeping you anchored and effectively eating most of the energy your muscles produce.
Your muscles are incredibly powerful. They can lift your entire body from a sitting to standing position with very little effort, a dead lift of 80lbs is so easy that it does not even register to you as a meaningful expenditure of energy.
In micro-gravity standing up with that same force would send you painfully crashing into the surface directly above you.
Any movement of your arms would add rotational energy that you wouldn’t be able to counteract without holding on to something that would anchor you and eat that energy. When you throw a ball if you lean into the throw then you might have to extend a leg behind you to act as an anchor and keep you from falling forward. In microgravity you’d just start spinning, possibly with a leg extended in a goofy way.
Since there is no resistance there is no way to slow down. It allows them to move around with that huge spacesuit. Fast moves can cause a big response. Any bump in the wrong direction may cause them to lose control. Therefore everything is in slow motion.
Good answers, but one factor hasn’t been mentioned yet that has nothing to do with reality: in movies and TV, it’s really difficult to capture the look of low- or zero-gravity on Earth, So many TV shows and movies will literally film the astronauts in slow motion to make things look a bit more “floaty”. This was especially true for older and cheaper stuff that didn’t have access to good CGI or a “vomit comet” aircraft.