Maybe they’re very good at it. We see them rarely and see them clearly or get good photos almost never but they’re actually all over the place. The ones we see are malfunctioning in some way or flukes caused by chance conditions of light, angle, etc.
Interesting points that I had completely overlooked!
By the way, without knowing how many there might be in total and how often they were seen/discovered, we can't really say if hiding from us is something they'd suck at :)
for instance, maybe they just care about the sea floor, and only surface to beam data back. or they're preferentially attracted to fast-moving objects like fighter jets, or sources of unusual EM signatures.
if UFOs are alien drones trying to hide, why do they suck at it?