You assume they 'left' vs just hanging out in Agartha, etc… until a harvest. Lather, rinse, repeat. Upgrade the good ones, recycle the dumb ones, start over.
You don't ships 'landing' to go inside. Just entrances and access controls and escorts. Reseal, deal with the surface infestation…restart round ? … our history is far older and more interesting than anyone can comprehend.
Spaceship Earth…the real story and better ride is on the inside, just like any ship. We're basically barnacles at the moment. 1st graders at best.
Look at Adm. Byrd's diary. Flugelrads are the Ariani ships that are descriptively identical to UFOs and can fly super fast and silently, but are not ET, but very terrestrial…internally based. So, maybe ships, but not from 'space', per se. Our makers are still here and we are the newbies. Babies.