The reasons vary, but ultimately its because they're dumbasses.
For some it's a pride thing, for others they think they're being dominant when really they're just pissing off the person they're sending it to.
I forget what the other reasons are, but there's an article out there somewhere from someone who asked a bunch of guys why they do it and most of them sounded equally brain dead.
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https://imgur.com/IqHwxnf https://imgur.com/jUql1