If guys are using 40kg dumbells they are not what is generally considered a serious body builder. I assumed from the OP's question, and the desire for 50kg weights that the workout would be beyond guys that are benching 120kg and doing flys or dumbell presses with 40kg. Those weights while impressive are not necessarily indicative of a serious body builder, so even though unstated, it does indirectly keep the serious lifters at places that do supply big weights. The people running the gyms are aware of this and make a conscious choice to appeal to the twice a week crowd. More likely to have a regular stream of new members with that group. I doubt that serious body builders in Osaka are any different.

I go twice a day everyday and although that is a lot by some standards, the 40kg dumbells are sufficient for a fitness oriented person like me. A body builder however, I am not.