Why isnt the offspring or progeny of Mirza Ghulam ahmed not Ahmedi prophets

The immediate offspring of Adam, Seth, was a prophet. Ibrahim had Ismail and Isaac as sons and prophets. Isaac had Yaqub and Yaqub had Yusuf. Dawud had Sulayman. From the progeny of Ismail bin Ibrahim we have Muhammad. (Peace be upon them ) The offspring or progeny of Mirza Ghulam Ahmed the Ahmadiyyah prophet aren't considered prophets but caliphs. Why is this distinction made? Given that he had 2 sons who reached adulthood and he was paternal grandfather to boys.