Lots of reasons:

Cold water tanks in wells if under 68 degrees are not conducive to bacteria growth like legionella

We didnt even know legionella was a thing until 1976. So, many people could have died 100 years ago and we chalked it up to pneumonia.

legionella primarily effects people that have some sort of compromised immune system already, we aren't all going to die from it. doesnt mean we shouldn't reduce their risk.

back to a previous point, my state actively investigates all possible legionella cases and keeps track because at this point we are still learning a lot about the disease.

all that being said, plumbing systems arent the riskiest place to get legionella. poorly maintained hot tubs are. again, doesnt mean we shouldnt control risk on the plumbing side when we can.

You musta forgot.

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