Why do my pops keep leaving my external factory?

There can be several factors like the location where they live, if the factory is too far from a dome or their moral going down. The most common cause is that the IA tries to fill the "important" working places first like grocery shops before production ones that are less vital. It also tries to balance all the working places to have the same amount of people in each of them meaning that if you have a polymer factory and an electric parts factory, the work force will be split between both of them

Working outside causes sanity damage. When the sanity of a worker gets too low, they quit their job and become unable to work. They eventually get better, but they will then take a new job.

Possible workarounds:

• Research "Martian Resilience" which makes Martianborns immune to the outdoor sanity effect and only assign Martianborns to these workplaces
• Research "Smart Housing" and assign your workers to smart houses/complexes. Their sanity recovery effect should cancel out the sanity loss from their job.