[Fyron]

There is a lot more than just the gases of the atmosphere. As Phoenix said, None-breathers require a vacuum. Planets with an atmosphere do not have a vacuum, they have lots of pressure. So, they need domes in which to create a vacuum on a planet with an atmosphere. Of course, many other things should be factors too, like Radiation levels, Heat, Gravity, etc. to make colonization more realistic, but those aren't modeled in SE4.