A water treatment plant would need about 2 people to a shift (and 4 sets of people) to have 24/7 monitoring (one to watch the control screens, and one to handle tasks like running tests on water, handling deliveries, etc., that takes you away from the screens), and that basically doesn't change if you're a small facility making 10KGD of water or a large facility making 100MGD of water. There is serious economy of scale going on here.
If you're a small facility servicing a few thousand people, you can't afford to have that kind of monitoring, and so you have to economize in various ways. One of the popular ways is pooling together with other small facilities so that you have one person doing that monitoring for several sites at once, which requires some form of remote operation.
Furthermore, when I worked at a large water company, all of our network, even the telemetry to the various pumping stations dotted around the service area, was on a private network airgapped from the internet. But there's also economy of scale here; a large company servicing 1.5 million people in a large metropolitan area can afford to do custom fiber backhaul in a way that even a bunch of small companies in the rural Midwest cannot, and so the control systems end up being Internet-accessible because it's too expensive for them not to be.