I would assume people would capitalize their titles in posts since that’s the standard pretty much anywhere else like on YouTube or news sites, and that’s what you learn to do in school. Why is it only different on forum platforms like Lemmy or Reddit?
My guess is the more casual nature of the communication going on. It’s less formal than putting in the effort of creating a published work like an article or video. Here you can just put the words in a box and hit go, your content is on the internet. Articles and videos require more time and effort to put together, so it would make sense to me for those to appear more formal, even if the content isn’t the most serious.