Since neutrinos are only subjected to weak interaction(ignore gravitation), thus the interaction of neutrinos are only governed by Z bosons, and photon is unlikely produced. If two neutrinos annihilate at an energy scale sufficient to produce a Z boson, then the Z boson can decay into various particles including photons. But what if the energy scale is insufficient for Z boson production? Does that mean the neutrinos will never annihilate?
Low energy neutrino and antineutrino can annihilate into three photons, but with an extremely low cross-section. So it's not "never", it's "hardly ever" (http://www.sciencedirect.com/science/article/pii/0029558266902331)