The threat is not from strangers, but from fully vetted and trusted community members having some kind of psychological event that leads them to bad actions.
The various entities working at McMurdo do pretty thorough pre-deployment psych evals to try to catch the kinds of people who would break vital equipment (think furnaces, or water-treatment equipment or high-cost/one-of-a-kind research equipment) or equipment necessary to fix vital equipment. But there's really no guarantee until you're out there that any given person is actually trustworthy. To say nothing of emergent psychological events that would drive a person to do such a thing. And anyway, the kind of person who wants to go to Antarctica frequently doesn't conform to our expectations of what a well-adjusted personality looks like. When your threat model is "trustworthy community members who occasionally and unpredictably become threats", you lock doors.
The various entities working at McMurdo do pretty thorough pre-deployment psych evals to try to catch the kinds of people who would break vital equipment (think furnaces, or water-treatment equipment or high-cost/one-of-a-kind research equipment) or equipment necessary to fix vital equipment. But there's really no guarantee until you're out there that any given person is actually trustworthy. To say nothing of emergent psychological events that would drive a person to do such a thing. And anyway, the kind of person who wants to go to Antarctica frequently doesn't conform to our expectations of what a well-adjusted personality looks like. When your threat model is "trustworthy community members who occasionally and unpredictably become threats", you lock doors.