Hitches are simply to heavy and too inconvenient to steal.
There is also very little black market for them.
There are not many people that need a 5th wheel hitch, especially a stolen one.
Thieves are more likely to steal the entire truck vs. taking a heavy awkward hitch.
I have a expensive air ride hitch, the last thing on my mine is worrying about someone stealing it.
If they can find a way to get it out quietly and easily they deserve to have it. But again it would be simpler and easier to take the whole truck.
Trucks get stolen regularly hitches almost never