There's a reason none of _those_ have been built either. No amount of red tape would make them unviably expensive if the end product was cheap enough to run, but it isn't.
Also I should mention, part of what makes them passively safe is there is a plug at the bottom that melts if the reactor overheats. This is easy to bypass, put something over the plug that won't melt.
Let me introduce you to the Molten-Salt Reactor Experiment[1].
What you probably meant is that none have been built commercially. That is true, but again as I mentioned, not because of their technical drawbacks but because of politics. In fact, the inventor of the light water reactor, Alvin Weinberg[2], was a strong proponent of the molten salt reactor over his own invention. So strong that he fired was from ORNL because he was claiming that light water reactors are inherently unsafe and that MSR is a better design.
Nixon ultimately sacked him because he (Nixon) chose to support LMFBR (Liquid Metal Fast Breeder Reactor) because it was being built in California, and in return he got political support that he needed. MSR ultimately lost due to pork-barrelling.
> This is easy to bypass, put something over the plug that won't melt.
I mean you're shifting goalposts here. The "operator" has a specific meaning - someone controlling the reactor from the control room. They don't have access to the freeze plug during normal reactor operation.
But even if they did do what you're suggesting, the pressures inside the MSR are so low (on the order of couple of bars) that the damage would be quite limited.
[1] https://en.wikipedia.org/wiki/Molten-Salt_Reactor_Experiment [2] https://en.wikipedia.org/wiki/Alvin_M._Weinberg
That's not true. There are many designs (e.g. molten salt reactors) where the operator cannot do anything to cause the reactor to fail.
The reason we're stuck with water reactors is mainly politics, and somewhat laziness.