It could be alcohol. For me, that is a way of shutting my mind down from time to time. Otherwise, the weight of intelligence could be overwhelming.
It could be the ease of not having to struggle very much to have your ideas accepted. You don't have to be sharp because you are in a mediocre environment.
Maybe it's a social thing: you can't be as sharp as you want because that would make it socially uncomfortable for you. It's easier not to be the asshole and just go along with the flow.
The anti-whetstone is what dulls your mind, over the long term.