Because it's indeed the impression the episode gives, and it's never really been the greatest fit with the Vulcan mythos before or since. The Vulcans are supposed to have the potential to be violent savages if they don't restrain their emotions; being actual practicing violent savages who just kind of bullshit about being "logical" without employing rationality where it would really count is not at all the same thing. Do we ever hear about this particular ritual ever again in any form of canon Trek?
: "Blood Fever"
It isn't a matter of society but of biological urge:
Amok Time wrote:
MCCOY: They still go mad at this time. Perhaps it's the price they pay for having no emotions the rest of the time.
The reason "challenge or death" still exists is because they have no other outlet for their aggression. Either you win the mate and get your groove on, lose and die which you no longer need to get your groove on or win and the aggression you get out kills the urge.
I think the episode and the rituals make perfect sense in a society where emotion is suppressed 99% of the time. Sex or violence in the absence of sex are the only acceptable outlets in their society. So they see the ancient rituals as still being "logical".