Board Thread:Naruto Discussions/@comment-1018594-20160409062530/@comment-734582-20160616100848

If you think about it, the Biju can reincarnate similar to how Ashura, Indra and Hagoromo and Hamura can.

Unlike those four however, Biju have no body to return to (the Gedo Statue is sealed and their connection to it broken, which is why sealing them in Gedo took 3 days). So they randomly respawn.