The Clan way of "warfare" worked great for
them, because its ritualized nature
heavily emphasized conservation of resources. Sure, 'Mechs were destroyed and Warriors occasionally died, but deliberately attempting to kill an enemy MechWarrior (aiming for the head, for example) was discouraged, and defeated units that retained
zellbrigen through the engagement were
always allowed to retreat. It clearly wasn't all that lethal, because taking bondsmen was routine and Clan Warriors also survived into their 30s and got dumped out of the frontlines in favor of the new generation often enough for the Clans to bulk up their
solahma infantry units with them. And keep in mind a single Infantry Star is
125 soldiers. If even only 10% of them are older or washed-up warriors, it's still a lot of MechWarriors and Elementals surviving likely dozens of battles before being forced out of the frontline Galaxies.
So the Clans got to settle their disputes by ritually shooting one another,
batchall bidding rules encouraged minimal use of force by both sides, logistics chains were extremely short and simple, and the vital infrastructure and civilian population in the sometimes barely-inhabitable Clan homeworlds were preserved even after being captured and recaptured multiple times. Even destroyed 'Mechs were either repaired or cannibalized, with parts being reused over and over again so long as they were still functional. The system did the job for them so long as they were all following the rules, since they were all technically under the rule of a single entity: the Clan Council and the ilKhan.
Then they invaded the Inner Sphere, who
didn't play by the rules. And the Clans invaded without any up-to-date intel beyond what ComStar gave them since the Wolf's Dragoons had bailed on their original recon mission 30 years earlier. And we all know how it went: predictably poorly.