Who thanks whom?
Just spotted this for the first time: Luke 17:6-10 and Luke 17:11-19 are deliberately juxtaposed.
17:6-10 establishes that the right way to relate to God is as his servants. When we serve him, he doesn't “thank” us, because we recognise that we are merely giving him (a tiny part of) what he is due. As the commentaries point out, this is really about the fact that God does not owe us anything because of the service we have given him. Our service never puts us in his debt.