Forums are kind of a relic from older times, before responsive web design was possible or even a consideration. I think that, as a result of that, the people who develop and maintain forum software haven't really started considering responsiveness until somewhat recently. And if the main developers don't, theme designers don't, either. ProBoards, for example, announced in 2015 their intention to make their software responsive by default. Until then, most of their userbase wasn't even familiar with the concept unless they had other forays into web development. Since 2015, I have only seen maybe four truly responsive ProBoards themes, two of which I coded myself.
The other issue is that forums are—by their nature—very information-heavy, which adds another level of complexity to designing a clean, usable interface without excluding important information. That means that the people capable of such an effort are probably professional designers or developers, and professionals are not often in the habit of offering their time for free. This is especially the case if the forum software isn't responsive to start with.
According to Statista, as of February 2017,
mobile devices accounted for 49.7 percent of web page views worldwide. And yet in 2019, I am
still finding whole-ass websites—nevermind
forums—that are not mobile friendly.