Unions serve a purpose. For example, Amazon demands that their employees do mind-numbing work only until they can replace them with robots, and a union stands a chance of fixing that.

That said, unions of public employees do not serve any such purpose, as the government ostensibly wants its employees, like all citizens, to be treated well, and the government does not have the same pressure as private companies. Even though some similarities exist, there are enough qualitative differences to justify any government to outlaw dealing with unions. Public employees unions routinely hold the citizenry hostage to serve selfish interests. The teachers' unions are prime examples, but for extreme examples look to the California prison guard union.