I'm not sure what they do when they have to open an airlock; I don't know if they have turbo pumps to evacuuate the airlock before opening the outer door. It may be easier to just vent the air to space, and then replace the air later?
On the subject of just removing the CO2, I think there are scrubbers to do that, but maybe you get all sorts of things in the air and it might be a good thing to vent some of it occasionally.
Imagine if you lived in a completely sealed house with plants to convert the CO2 to O2. What about the other stuff? You know the gasses you get depending on what you eat, and the poisonous fumes given off by all that Ikea furniture as the glues and other things used to make it slowly evaporate from it?