Neither do the other thousands or millions of people who do so, but it’s not a particularly uncommon thing and it’s likely enough that (birthdate) is the reason it’s there. I’ve got my birth year (not '88) in my username for some sites because the bare username was already taken

He’s a money supporter.

Yeah, birth years are super common in usernames (and password, don’t use it there). Bob Smith finds his preferred username is taken so becomes BobSmith79 or BobSmith88 or whatever because it’s easy an easy enough variation to remember.

You can find patterns/relations in almost anything if you reach for something, kinda like the “six degrees of separation” thing even if there’s a more reasonable answer

They did. Not many choices left that support it, sadly