Simply put, because we don't have a perma-ban feature that just says "this account is banned forever" with no timeline for when it ends. We never thought we'd actually need something like that, and cases like this are still incredibly rare. At least rare enough that investing time in another type...
The tag excerpt for hidden-features mentions the post timeline as a hidden feature. It was written back before the link to it was added, and is now outdated. Any other good examples that are a better fit there instead?