As with any group of entities, all have an agenda. Over the eons,
literally thousands of alien types have visited Earth, having one agenda
or another. Some agendas are conquest, though these are in the minority. A
great many agendas are curiosity, and these are never seen as harmful.
Some agendas are beneficent, but these must never interfere with the
free-will of the resident, natural intelligent life forms. Where you and
the hominoids residing on the 12th Planet are both operating in the 3rd
Density, they mastered space travel enough to shuttle from the 12th Planet
to Earth during the periodic visits. They cannot shuttle to Earth at this
time, as it has been deemed by the Council of Worlds to constitute
interference with your free will because their interaction with humans
sprang from their agenda for conquest. In times past, other rules were in
place and interaction did occur. The quarantine was put into place some
thousands of years ago, but close enough in your recent past that contact
with this alien life form remains in your legends.
They have no plans, despite fear mongering among those claiming to be in contact with these folks, to return to Earth, which they consider a swamp. The Earth in their memory is full of carnivores, rebelling slaves, and accidents related to wild weather. Now that mankind has technology, a fact that cannot be denied due to probes sent about and requiring their mining operation on Mars to shoot them down when they come too close, they are even less inclined to visit. They were on top before, but do not anticipate winning a war on Earth today due to numbers. In addition, what would the gain be? Do you live in a swamp when you can live in a palace on the hilltop? This was not a vacation resort for them, but going into the field on a mission, not the chosen one either. The inhabitants are interested, during this passage, in collecting the gold mined since the past passage, and in watching the light show. However, one of the legacies left on Earth by the hominid visitors from the 12th Planet is radioactivity from their nuclear warfare. Yes, they had the bomb, and used it. In attempts to nuke each other into compliance, they dropped numerous bombs, the majority on the Arabian Peninsula, with a small number in India and a scattering across the African Continent. These bombs were in the main not as large as the ones built during the Cold War, but were more deadly as the radioactivity lingered and poisoned the ground, making it utterly uninhabitable, for millennia.