Because personal expectation level varies based on the person and when they are introduced to players they are placed with at a random queue it may not be as one expects. For example, if one person performs well as a A rank while another person performs well as a A Rank but does not perform the way that other person would have done things then they would not have met that other person's expectation level.
The Rank system only would score them based on a set system expectation level and how they achieve that level of Rank varies on each player.
On Topic:
There is also different personality of the player to consider as well with as even those who perform at a S or A Rank level of skill can still have a toxic personailty. As much as the OP believes the toxic personailty would go away or moved to low tier rank only groups in a Rank system it will not go away for a S, A, B, and etc rank as there will always be players who can perform at the Rank or a S, A, and B who have a toxic personailty despite having the skills of a S, A, and B Rank player. There are still plenty of players today who would be considered S, A, and B Rank Players who still quit a dungeon over "New Player" and/or 1 wipe due to a accident happened on another player's side, such as life emergency or DCed through different connection issues, or even if the player themselves made a mistake but does not want to acknowledge it such as not communicating with the group on how to handle bosses ,checking if the entire group is ready, assuming the group would skip a group or attack a group, and etc.
Communication is a vital part of a group that can perform well but that is something that cannot be ranked.
Then there is also issues of differences in Job's ability to perform their roles. The most being within the DPS rank as Jobs like Machinist and Bard still remains a bit lower than the other Jobs on DPS even if they perform at their best while jobs like Summoner and Dragoon can perform better on their DPS with less effort than a Machinist and Bard. The calculations will have to take into the Job's individual Min and Max DPS ability so it can rank it properly as well which can be difficult especially with "balance" issues that Jobs suffer from even now such as Bard, for example, had to under go a buff recently to perform better.
Healers Ranking would also have to calculate between DPS and Healing as well due to times when the Tank can perform well without losing a decent amount of HP and times when more DPS may be required. Scoring systems can be a bit crazy when Scoring Healing as a player may spam heal a lot to build up the Healing points even when groups do not need it.