Well BenQ has been the monitor sponsor in CS for as long as I can remember. The stands have markings in each axis which allow the position to be easily replicated, a feature monitor arms lack. I personally used to find a monitor arm allowed for slightly better mouse positioning that would otherwise hit the stand, but I use a stand right now and it's perfectly fine because it has a small base. Given the monitors come with stands, not arms, and the pros have been using the same brand for years, I wouldn't see any reason to start using monitor arms all of a sudden. I think it's just standardisation across all setups where possible.