Not to mention their faster than light speed communications back to their home planet. And how they managed to find us in the first place. The very first long distance radio broadcast was made by Marconi in 1901, so is now detectable in a sphere with a 123 light year radius centered on the Earth - inside the green dot below. Unless the aliens were already inside this sphere, they wouldn't even know we were here.
How Far Have Our Radio Broadcasts Traveled in Space?
Proponents really aren't doing themselves any favours either, as in the case of the former Pentagon official who ran the "Advanced Aerospace Threat Identification Program", Luis Elizondo, who last month revealed this "classified photo", which he claimed to be of an alien mothership seen from the US embassy in Romania. It turned out to come from Facebook, and is a chandelier reflected in a window miles away from the embassy. The fact that Elizondo has written several books on UAPs, and revealed the photo at a paid event in Philadelphia, may tell more about why he's so interested in producing this "evidence".
Pentagon chief reveals high-res photo of a UFO 'mothership': 'A huge mini city floating in the sky' | Daily Mail Online
Given that these days millions of people carry around high resolution still and video cameras in their pockets, I'd expect at least a few civilian photos of these UAPs to have been released on social media by now. I can take a photo of a plane flying over my place at 10,000m with my phone and see what airline it is, so where's all the high-def images? All we get is "swamp gas from a weather balloon was trapped in a thermal pocket and refracted the light from Venus" style blurs and blobs. I'll be more than happy (or maybe not, depending on their intentions) to be proved wrong, but I see Earth as more the type of neighbourhood that any self respecting alien would drive by as fast as they can, while making sure their doors are locked.