Board Thread:Naruto Discussions/@comment-32617252-20180630010918/@comment-3568954-20180630050946

they are more than just pure chakra, they have flesh as well. Hagoromo divided his mothers chakra into 8 sections than used creation of all things to breath life into each of them.

moreover if the biju absorbs natural energy and mixes it with his or her chakra than its already sage chakra. if the biju was 100% just chakra than once the biju lost its chakra than it would disappear