they came there to rob and murder you all and the only punishment you give them is you spread their favorite horsehair? i don't care if the horsehair and saddle is sacred to them it's not enough.
but in other country, they will comes back even more
The real reason is that this is a more family-friendly type of manga focusing on little kids, so not killing is rather justified in that sense. However, from an in-universe perspective, these nomads are now marked as having done something shameful. Other groups are less likely to trade with them, and will generally treat them poorly, at least until the hair grows back and they can replace their saddles. The rest of the tribe at home is also likely to treat them badly. Not only are they marked, but they didn't bring back the food and money. The pressures on the tribe that drove them to this attack (greed may be a powerful motivator, but hunger is even moreso) are still present.
If the village killed the raiders, that not only would reduce the pressure on the invading tribe rather than continuing to have to feed the failed raiders, but killing them would also invite retaliation. The last thing that the village needs is for the entire tribe--possibly along with allied nomadic tribes, some of which were previously valuable trading partners--to wage a more serious war, with the goal of not just raiding, but exterminating every last man, woman, and child.