Any reason why they all can't come together and standardize on the plug?
If you are doing things like plugging into an RV power outlet at a campground, sure. For normal people using public charging stations, no, because...Do people with EVs have to carry an adapter in their trunk to make sure it works with available charging station?
They have, regionally. China has a mandated standard. Japan prefers ChaDeMo. And in the US...The PC and smartphone world standardized with USB C, so I think it'd be a lot easier if the various OEMs could do the same for the ports.
They realise this, which is why in the US...They'd save themselves a lot of headache by going that route.