In an environment where you presumably know every single human on the site and there are probably no other humans in any direction for 1000 km, is there a need for locks on anything other than bedrooms and bathrooms?
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.
I like to think that a spoof "How to Pass the Psych Eval" video we made one winter might've somehow contributed to that particular system being torn down a few years later, though I don't have firsthand experience with the system that replaced it. My understanding is that the new one involves more "team building" type stuff before deployment, which sounds like a step in the right direction.
One issue is that by the time a person has gotten to that late stage in the hiring process, they are often deeply invested in deploying - having packed things in to storage, quit "real world" job, etc. and their nominal manager understands that. Another issue is that people behave differently in different environments, and Antarctica is quite unlike Denver.
to keep the Thing out
(/s, obviously...)
Also lots of standard doors come with deadbolts pre-installed or at least a deadbolt hole drilled through.