So what?  The inability to prove the non-existence of something is in no way a point in favor of its existence.  The rational time to believe in something is when we have reason to believe it exists, not when we lack proof that it doesn't.

Because you cannot prove there are no Leprechauns, there exists the possibility that there is one.

Because you cannot prove there is no Bigfoot, there exists the possibility that there is one.

Because you cannot prove there is no Babbadook, there exists the possibility that there is one.

Do you believe in Leprechauns, Bigfoot, and the Babbadook now?  Somehow I doubt it.