It's probably a much simpler answer. Why set up a bunch of plumbing and networking inside a dam if you don't need it? Many (most?) were built before the internet, as well. And I don't think they generally are staffed around the clock. Any more complexity than you need is a waste of money.
I bet it's exactly this. You've a massive concrete structure where adding water lines and power is going to require quite a bit of paperwork before you can even begin to consider how you run it. And they can be quite long, too, and your sensors likely need to be somewhere that power (and clean water) currently isn't.
And then you will discover just how well wifi travels through concrete whose thickness is measured in meters.
