It strikes me that from a company point of view, they may want to project an image of professionalism in their corporate image, hence pilots wearing uniforms when they go to work, whether in the flight deck of an airliner, or a simulator.
Sim centers may have other corporate visitors, and I guess it could be argued that having pilots wandering around in casual clothing may not project the image/illusion that an airline may be looking for. Speaking as an old fuddy duddy in their early 50’s, and passing on what I see down route in hotels and business lounges, it may not be a bad idea. Some younger pilots views of smart casual may not align with an employers or hotels view of smart casual. In which case, why not stipulate a pair of dark trousers, white shirt and a tie to be worn when on company premises and on duty? When on the companies dime, I guess they can tell you what to wear.