To further break it down in a way you can understand, as it pertains to petroleum being transported through pipelines, humans calculate the initial pressure needed to send the petroleum from say NY to TN but there's booster stations along the way to help it maintain a certain amount of pressure for it to reach it's destination. The reason why these booster stations are unmanned and SCADA is so useful here, is because it can dynamically change the pressure needed to keep that petroleum flowing through the pipe without causing it to burst or it can detect issues in transportation and close valves automatically as needed then send an alarm to a designated SCADA team. Humans cannot calculate the changes needed in pressure that quickly with tons of oil moving that fast. It's literally impossible.

From an economics stand point, your idea of having fully manned sites for all these companies, would cripple them financially and you wouldn't even get your oil or water in the first fucking place because they'd barely be able to afford to keep the lights on. A senior fluid dynamics engineer costs about 100k a year and a SCADA engineer is about the same and you're talking about having a whole team of these dudes on every site. Might not literally cripple them but quality in other areas would suffer because the oil companies and water companies would cut costs somewhere to compensate for that.

Also I never claimed that a nuclear power plant was literally the same as a water plant. Learn to read. But water plants do unintentionally create toxic chemicals through the treatment processes. Look it up.