I think the point is to have the viewer at least try to read the long version. If they can't or won't, they can quickly scroll down.
TL;DRs at the top are just like attention grabbing headlines and opening paragraphs that often don't tell you the whole picture, either on purpose or simply because it's impossible to do so in a sentence/paragraph.
TL;DRs at the top are just like attention grabbing headlines and opening paragraphs that often don't tell you the whole picture, either on purpose or simply because it's impossible to do so in a sentence/paragraph.