This is due to the curse of Goddess Parvati to deities. This is present in śiva Mahapurana Kumara Khanda chapter 2.
O Gods, particularly wicked, selfish and cause pain to others. O Gods for the accomplishment of the selfish motive, you by the supreme lord Siva, have obstructed my conjugal pleasures, making me barren at the same time. No one can feel comfortable by opposing me. Therefore you will also have to suffer. Thus spoke the daughter of Himalaya enraged, pronounced a curse on Gods. From Today onwards the wives of devas would remain barren (without children). Such of the Gods who will oppose me shall suffer.
In addition to this curse to the Devas, She also cursed Agni.
O Agni, from today, you shall be the consumer of all (sarva bhakshaka). You will also suffer pain.
This curse was pronounced by Pārvati after knowing that Agni swallowed the semen of Lord śiva. So the count of the devatas was only 33(not 330 millions) when Gaṇeśa and Kārtikeya were born and it is 33 now also.