Because they can control who is on it, they're journalists only, and still be out in the open with no sign ins. What would be the benefit of them joining other instances? That would be an odd choice.

Because then someone else would be able to control and censor their content. Really every business should make their own server to ensure that they're the ones fully in control of their content - this is the entire point of federation.

I think the USA's National Weather Service Twitter presence is a good example.

If you look deep enough you'll see caveats like "supplemental service provided by NWS" and "Twitter feeds and tweets do not always reflect the most current information", but the truth is that a lot of people (and news organizations) depend on Twitter as their main interface to the NWS, and rarely if ever go to their website.

That obviously creates a tension, which bubbles up in scares like this:

Before last weekend’s storm, the National Weather Service’s Baltimore-Washington office sent this tweet saying that because of a new Twitter policy, automated tweets that show advisories, watches, and warnings might not load.

Contrast that to a world where NOAA (the federal administration which runs NWS) has their own instance: they get the benefit of being able to disseminate updates in a consumer friendly 'social media' style and they retain full control of platform and can be sure the service won't be held hostage, or go down in the middle of a storm.

Finally: if you're reading this from the USA, consider contact NOAA/NWS to let them know you'd like a fediverse presence, I did!

Having their own instance as a public organization adds more legitimacy to their publications. Think of government officials using the organizations domains for email instead of gmail.

Them having their own instance would serve the same purpose as being verified because of the domain.