Twitter prioritises concise, lightweight discussion. Instagram is all about bitesize visual communication. It's fast, breezy interaction reaching a massive audience. The expected types of content and modes of communication in both are very succinct, meaning it's highly efficient for a community team to engage in at scale.
The forums are the land of lengthy personal manifestos, immortal threads, and cliffs of text so high there's puffins nesting on some of them.
To engage on the forums in an "equal" manner to other forms of social media, means wading into much deeper weeds and sinking much more time to engage more closely with a much more verbose and demanding audience. Talking to a CM on twitter is like asking someone at a company helpdesk for information, whereas on official forums ("company property") people tend to feel and act like they're sitting in the company boardroom. Add to that the far, far smaller reach of the forums and the fact that most of the audience here is already very engaged, and it's pretty obvious why they prefer outsourced social media platforms to their in-house forums.
I'm not saying that's how it should be, but it makes perfect sense why they would tend to do this stuff elsewhere
