Hi there, thanks for the reply.
It’s a large forum that was migrated from a traditional forum, so those older conventions stick. If it’s a fairly short post that is quite far back in the conversation thread then some might quote the whole thing if they’re replying to it, thus helping preserve the threading (although Discourse has its own automatic reply threading). Also some mobile users have commented that it’s easier to just quote the entire post with a single button click, rather than trying to finger-select text and deal with the popup quote button.