We humans unconsciously assume "they" may be remotely like us. That is almost certainly an error. They are most likely not like us at all. It's possible that some “objects” we see that display impossible maneuvering abilities aren't ships but living beings that are not corporeal as we experience it. I'm speaking of some of the 'balls of light' objects. They may be capable of travel in a way we do not understand at all.
The ones that look more metallic and substantial may be probes. Their sizes and shapes may relate to propulsion rather than cabin space. I don't think beings waste time coming here in ships, flit and leave without even stopping by for coffee. They know all they need to know about us and we don't even know if they are secretly guiding us somehow or just studying us like we study germs.
The other thing I consider is that if indeed we have been visited by extraterrestrial civilizations it isn't the rule they'd be friendly. In nature the smart animals are the predators. They have to be smart, cunning and ruthless. Sending out the "Hello, here we are!'' might just be ringing a dinner bell.