Mostly because the people writing them don't understand how their post will be parsed, and the post lives simultaneously on Lemmy / Kbin and Mastodon.

Lemmy just takes the first paragraph from the post as the title. So if the poster knows that, they can format it like:

Title title title

Body of post

I think it uses the tweet content as both the post title and body when Lemmy tries to display masto tweets. Maybe the title should just be "Mastodon post from @username" tbh