Officially, they don't take it because they are employed in a Non-Tipped Position as defined in company policy and under their collective bargaining agreement and risk immediate termination from Walt Disney Parks and Resorts U.S, Inc. if they are discovered to have accepted tips.
Unofficially, many Housekeeping Cast Members don't speak English or have a very limited command of it. To someone in that situation, "Mousekeeping" might as well refer to some sort of Animal Control Agent. Also, if it isn't somewhere where it's plainly visible, they might just miss it as they rush to clean the rooms.
There are a few Cast Members who do follow the rules and just don't take tips. In those cases, nothing you can do will get them to accept it.