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CISA Releases Guidance Urging Water Treatment Facilities to Disconnect Equipment From the Internet
(www.privacyguides.org)
This is a most excellent place for technology news and articles.
You're talking a massive nation with more systems like this than you could ever imagine. Probably more than all of Europe combined.
They are in every state of repair and disrepair that you could imagine, and many have been receiving government funding for at least at decade now (def more, but I can only speak to the last ten years or so) to replace these systems, including SCADA and PLCs.
So you have a patchwork of countless systems, in every level of tech modernity that you can imagine.
The newer systems usually have an internal network of sorts, and they might have a portal for someone outside the network to log in to remotely monitor.
The problem seems to be that these idiots kept the default credentials, etc.
But to answer you're question, they're not all 20-30 years old, and network connectivity (without being open to the outside internet, obviously), is a very useful feature when replacing those old systems with newer ones.
That's why.