Hm, I guess this will require a full article to respond to it properly, but in short, I don't think that you, and many skeptical scientists, fully appreciate what is required to be able to study an adversarial subject, or, well, objects.
Rocks in space, or even alien debris or publicly broadcast signals, will let you observe them, while the human militaries and intelligence communities will also let you observe those objects.
You have to factor in the possibility of alien observers not wanting to be pinned down with scientific certainty, like for example to not interfere with our development too much, and the confidentiality and diversion caused by militaries, that see those objects as threats they can't address, and by intelligence agencies, that have cells within them that will want to hoard any potentially gainable technological advantage for themselves.
Every single kind of evidence you may want - crashed craft and exotic materials, alien bodies for autopsy, radar readings and credible and mass observations, abductions and communications, insider whistleblowers - has been at least repeatedly alleged, while most of them are being repeatedly documented and brought forward.
The adversarial nature of the subject requires working hard, with initiative, to make sure that the evidence that exists is even fairly evaluated, let alone not snatched away or destroyed by a covert operative the moment it becomes evident it is, in fact, too incontrovertible.
You can call people who argue like this paranoid, and I can call people who argue like you naive or shortsighted all day long, but that is another thing that only makes it less likely that phenomena like this will ever get figured out scientifically.