To say that there aren't antineutrinos feels somewhat incorrect. It isn't that there aren't antineutrinos, Majorana particles are their own anti-particles, as such neutrinos are also the antineutrinos.
The reason why the lepton number conservation is necessary is because some reactions like the Neutrinoless double beta decay (0νββ), Neutrinoless double beta decay, have yet to be observed, one way to justify this is by assigning lepton numbers and guarantying they are conserved.
Finally, one last thing that could help you is knowing that all neutrinos observed are left-handed, while all antineutrinos are right-handed Sterile neutrinos , and as such if neutrinos are indeed Majorana particles, the neutrino lepton number (+1) would be assigned to the left-handed neutrinos, while the (-1) would be assigned to the right-handed neutrinos.