Why does an astronaut?

Because they are falling. An orbiting spacecraft and everyone inside it are pulled by Earth's gravity at practically the same strength and accelerate together, so nothing presses on anything else. There is no floor pushing back, and weight, which is what we actually sense, disappears. Gravity has not gone anywhere: at the height of the International Space Station it is only slightly weaker than at sea level. Astronauts float for the same reason the occupants of a falling lift would, except that the spacecraft's sideways speed means the fall never ends.

Outside the hull there is nothing to breathe and almost no pressure. Unprotected, the gas in the lungs would rush out within seconds, dissolved gases would come out of solution and water in exposed tissue would vaporise; consciousness would go before any of it finished. A spacesuit is therefore a vehicle rather than clothing: a sealed pressure vessel with its own oxygen supply, carbon dioxide removal, water cooling, radio and micrometeoroid shielding, and it must still let the hands work for hours. That last requirement, not the vacuum, is what makes suits so hard to build.

This is the version worth arguing about, since robots are cheaper, lighter and do not need to come home. The case for crews is not strength or endurance but judgement: a geologist on a slope decides in seconds what a rover would need days of ground commands to attempt, and the Apollo samples that reshaped our picture of the Moon's origin were chosen by people who understood what they were looking at. Repair is the other half, as Hubble showed when it was rescued and rebuilt by hand. Against that stand cost and risk, which is why sensible programmes send both.