This is a really interesting question because I've wondered the exact same thing. I participate exclusively in PSL's, so the username is generally just a necessary formality in that instance because I've been writing the same characters with the same person for, like, six years (I didn't even realize that OOC usernames were even a thing people did until I came to IJ, and by that point I'd been writing the same characters for so long that I just remade the majority of them with similar IC names).
If I were to hypothesize, I'd probably just tack onto what paulus stated--peer pressure is probably to blame; my first game had an unwritten "rule" about choosing an IC username--it was a fandom game and the creator had used the character name--and I think the rest of us just followed suit. I wish OOC names were more popular when I started, because I would have loved to get creative (there's only so many variations of a person's first and last name!).