The cost of rebuilding the site as an app would be insanely high (ie as the forum is built with XF, not something conducive to cheaply making an app like if it was built in React/RN).
What that means is building the app(s) from the ground up and integrating them with the data flow from the forum = a huge and costly pain in the ass that would take 6 months at best.
Other platforms that do something like this have an incredibly non-mobile-friendly desktop version so users are actually unable to use the site effectively using mobile. That's a no brainer then - an app makes sense a lot of the time.
But when you have a site built with a framework as mobile-friendly as XF2 is (inherently, they've built it really well for mobile usage, moreso than any other forum framework) then the value in going to the effort and expending the cost of building apps for iOS and Android really isn't there.
I don't know how many new users would find the forum via apps that wouldn't have found it through Google or other communities anyway. I doubt there are tens of thousands of people casually searching for "blackhat seo app" on the AppStore each day.