The reason is quite simple. Most power plants are in the middle of nowhere and people with the capability to manage them don't want to live near them, not because of the plants themselves, but because they are in tiny rural towns, which most people understand and appreciate. To make good decisions, the experts, who prefer to live in metropolitan areas (like most people do) need access to the data these generators produce. If the generators weren't on the internet, that'd be impossible.

TLDR: Power stations are on the internet for the same reason everything else is on the internet.