We all know Muk and Luk's mother either died, abandoned them or just got seperated from them but polar bears NEED their mothers to survive. Without the mother, the cubs are as good as dead, unlike wolves who look up to the pack for protection (probably how Balto survived, Aniu wouldn't be a lone wolf all her life after meeting Balto's father), polar bears are lone animals and teach their long until they are fully grown (for three years) but, until then, they are truely dependent on the mothers.

Yes, I know Boris maybe found them with Balto but Boris is NOT a polar bear mother and cannot teach polar bears the skills they need. I also noticed that Muk and Luk do NOT eat in all three films. While other characters display signs of eating or hunger (i.e Steele eating the sausages, Kodi and the others eating dog food in the third film), Muk and Luk do not and they do not mention they are hungry or anything. Then again, I suppose none of the characters really go for a toliet break either.

Just my point: Muk and Luk are cubs (and strangely remain cubs for the whole series) and cannot survive without their mother.