Even assuming they have some non-crazy motivation, instead of making the subjects fight in a cost-effective abandoned warehouse, the unlucky employees are hired by a fake company, relocated to Colombia, given competitive salaries, and probably each housed in a chic two-bedroom apartment with a washer and a dryer. The office building itself is equipped with an extensive surveillance system and massive steel plates to keep everyone in. To prevent the workers from signaling for help from the roof, a plethora of armed henchmen (who probably have chic apartments too) are stationed outside to stay inside and kill each other for science. Hidden murder-prisons aren't cheap.
And then, at the end of the film, we learn that there are at least 130 similar office buildings around the world, where other unlucky employees have beaten each other to death with fire extinguishers. All of this to learn lessons about humanity that could be learned by opening any history book.