in a BO5 grand finals, that one map (that may not be in your control to ban) may prove the difference and crown someone champion that should not have been it.
Also many teams have been working with a permaban on the same map for months on end. What do they do now? Ban breeze and leave their permaban untouched? Ban their permaban and be forced into an unfavorable matchup on Breeze?
It leaves too much to the question. You're only seeing 1 part of it but there's much more that you're not addressing.