The sum of all partial pressures in the lungs must add up to the ambient pressure.
The composition of gas inside the lungs includes the vapor pressure of H2O at body temperature which is about 0.9 psi. This is independent of ambient pressure.
Inhaled air is diluted by this H2O vapor. It is also diluted by the dead space gasses from the previous breath, which is about 150ml out of every 500ml breath.
If you tried to breath pure O2 at 1 psi, your lungs would be full of H2O vapor (steam). Your respiratory efforts would be useless as the H2O vapor would shuttle back and forth in the dead space and no O2 would get to the alveoli. However, you would feel only mildly short of breath until you lost consciousness. This is because respiration is driven primarily by blood CO2 levels, not blood O2 saturation. Your respiratory efforts would continue to eliminate CO2 despite being unable to oxygenate your blood.
To compensate for these effects, space suits use ~4 psi O2.
https://medchrome.com/minor/anaesthesia-minor/lung-volumescapacities-and-dead-space/
https://vrchemistry.chem.ox.ac.uk/potential/Text/solutions3.htm
More detail is provided by the excellent answer to Why is the EMU space suit pressurized to 4.3 psi specifically?