Relevant GitHub thread:
https://github.com/ppy/osu/issues/24238tl;dr: this is a long standing casualty of trying to simplify systems (in this case, making ranks depend solely on accuracy).
A few alternative directions were tried and people didn't like them (one being trying to move the PERFECT score bonus to actual bonus score, which ended up violating the sacred invariant of 1M score being the absolute maximum), so this is what lazer ranked play went live with.
Since then ranks no longer solely depend on accuracy, but fixing this
now is invariably going to be more difficult to fix now than before, because there are existing scores that will need their ranks recomputed.