Tents are designed to let air in and out, otherwise people sleeping in them would suffocate. A body bag is made of thicker material, intentionally designed to keep pathogens from escaping a dead body, but just as good at keeping pathogens out once sterilized, for the desperate. A tent could be disinfected to reduce chance of infection, but it would naturally degrade over time with exposure to the pathogens in the air, whereas a sterilized body bag would be permanently sterilized on the inside until opened.
Basically, using tents realistically would be too much programming work. Body bags are already there, and presumably the autodoc deploys an oxygen mask so the patient doesn’t suffocate during surgery. Of course, we could also just assume the autodoc includes a sterile tent that it deploys for surgery, and re-sterilizes afterward, and leave things as they are for now, until it comes time to revisit NPC and self surgery.