The simplest way to explain this is that your galaxy is contained. There is a kind of shield that will prevent travel in or out of the galaxy by physical beings and physical spacecraft. Even using wormholes to traverse great distances will encounter this resistance barrier and it will return them to their origin. This is seen to for reasons of safety. We do not operate by torturing and killing to reinforce our rules but have more elegant means to do so. What this means in a practical sense is that the extraterrestrials, including the Anunnaki, have been thwarted in their attempts to travel outside the galaxy and this is an area of frustration for them and a kind of “scientific blind spot” as they perceive it. So although they have worked on this to a considerable degree wanting to expand their reach, they have heretofore encountered this kind of anomalous behavior of the energies that represents a limit on their ability to move beyond the galaxy.
Being atheists, they cannot even envision the possibility of a higher power creating this enigma for a higher purpose, so it is just one of many “scientific curiosities” they have been unable to unravel. Despite their brilliance and their technical prowess on many fronts, there are a number of blind spots, still, in their understanding and awareness of larger possibilities. This restriction is imposed on your galaxy solely for safety reasons to protect the universe from corrupted beings who can harm others as a consequence of their departure from a divine path, which has happened to many groups of beings, courtesy of the fallen angelics who now fear the light and are corrupting many beings with their dark motives and perspective.