People don’t use external storage, because of how the forum handles attachments. People use it out of habit and nowadays also beause they installed a smartphone app sending images to such services. They take a photo and use that app regardless of what they do, to where they send, and why.(1) So the entire part about how forum works is IMO tangential to the issue.
Somebody earlier mentioned self-hosting. I’ll skip the entire part on how self-hosting is not a solution for most people. Instead let me focus on the much more important problem: not addressing this thread’s topic. Did you people wonder, why you didn’t see me linking anything from my servers? Because if tommorow I die, in a few months the content becomes unavailable and in a few years the domain is under control of spammers. That’s quicker than any image hosting service, even the most questionable one. If I have to face regulators or a troublesome VPS provider, I have much less options to fight back than a big service has. If your country or ISP decides to do something silly or deploy censorship, I’m out of luck. During site moves or restructuring I may forget I’m permanently sharing something. This did already happen to me years ago and so it did many times to other people self-hosting content.
If post integrity and stability of the content are the goal, self-hosting is not solving anything.
(1) To share some scary stories: lawyers sending court document photos and medical personnel sharing photos of patients’ data. In both cases not having any mental model of what’s happening. To the users it was “oh, I always take photos like that and it just appears on friend’s screen.”