Part of the reason is that people doing it don't put EOM to terminate the title. But the other part is that MUAs love to truncate the subject line for display because "UX", and the most popular ones (like GMail web interface, or Outlook) are janky - so in the end, what happens is, you have to open that email anyway, and then you see just signature with nothing above, and the obvious first thought is, "that stupid email client broke again", followed by "or the sender accidentally clicked 'send' before writing the message", both of which are much more likely than someone using subject-line-only message.
Doesn't also help that most normies can't communicate to save their lives, so the title is gibberish and full of typos. There's a minimum volume of text you need to error-correct that, and subject line alone isn't enough.