I have not even noticed that I do allow it or not 😛 something that doesn't even come to people's minds and whatnot. It's a yes anyway on my end. rules a basic common sense
I can see why they wouldn't because it uses up disk space and there's no real benefit to it.
But, if you run a forum where people are going to befriend others and it'd be the main point of their contact, I'd say it's essential. Otherwise, I might eventually just drift off to a chat app like Discord and convince others to do the same so I can freely attach as many images as I want to.
I personally have this option turned off, since I don't see any real benefit in having attachments shared between a very, very limited subset of user(s) on my community compared to being shown to everyone. At that point, I'd just recommend them to use third party image hosts like Imgur.