The answer is that 1. They’re privatised but 2. They’re effectively monopolies. The argument for privatisation is basically that private companies need to innovate and keep prices low in order to compete in the free market against rival companies providing the same service/product — in practice though unlike something like electricity (which I also think should be renationalised) you don’t actually get to choose which company supplies water to your home, it’s just based on where you live and there’s only one option, so soon each catchment the water companies don’t actually have any competition.
They don’t need to innovate because the consumers don’t have the ability to switch to another supplier (unless they want to be moving house constantly), equally they have no incentive to keep their prices low.
So they’re effectively the worst of both worlds. If they’re going to be private sector you might as well do it properly by creating actually competition, otherwise just make them public sector.