I think having a way to delete accounts is legally required by some jurisdictions. And sometimes if a site does business in such a jurisdiction and are required to have a way to do that, they'll still offer that option those outside the jurisdictions in question. (It's easier to just allow everyone who asks than to have rules keeping track of who can and can't legally demand it.)
But if this is an image board hosted in Japan intended for a Japanese audience, and if Japan has no such legal requirements (or if such requirements don't apply here for some reason), then, your experience with websites that operate in/for countries where they speak your language(s) notwithstanding, it's highly plausible this site just doesn't have any way to delete accounts.