It is a claim they inherited from the Qing dynasty which legitimizes their territorial claims. The current "reincarnation" process was formalised by the 5th Dalai Lama in agreement with the Manchu Emperor, and its origins are certainly more polotical than theological[0]
Of course Your opinion may differ but I it is no more absurd than the President of the Fifth French Republic receiving the title of the Co-Prince of Andorra alongside a Catholic bishop.[1]
