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[...] Mary didn't really try to engage with Clark [...] Mary was right to be terrified when entering the Backrooms, being a reflection of his mind, because she hadn't really entered it before.

I think she does it because of her mother. Her mother was delusional, and you can't entertain those delusions at all when interacting with them. Clark is angry and shirks responsibility, but he isn't delusional. Mary, stuck in her own loop, treated him like she would her mother.

It's probably (at least partially) why she smiles at the end during the interview/interrogation. She's finally understanding what it's like to have someone try to analyse you without really engaging. It's patronising and probably going to make everything worse. We even see a Still Life of her sitting utterly terrified and distressed in that exact chair, suggesting that was how Mary was feeling, while all that Phil had to offer was questions he already knew the answers to and a vague threat that she'll be taken away.

All empty, devoid of life, and stripped of its complexity............