The issue is just how many samples have been collected
You've raised this objection many times, but when I think about it, I can't recall any specific cases where soil or plant samples have been collected, can you? I can recall some cases where a device has landed and left landing pad impressions, and I can recall a few cases where trace residue evidence was left behind, but I can't recall any cases where we've found a lump of soil missing. I'm not saying that it hasn't happened; it's been ages since I've pored over case reports, but you seem to think that millions of cases involve soil/plant samples being collected, and I just don't see any basis for that.
If abductions are actual physical events, then I suppose we could assume that lots of biopsies are being conducted on people. But if that's the case, there could be lots of logical reasons for that, some that we could speculate about, and others that would be inscrutable to us without understanding motives - and that's a dead end because understanding alien motives is a hopeless task, imo.
In any case, the ETH implies that hundreds if not thousands or more alien civilizations could have the capability of reaching the Earth, and each one could decide to study our world and the ecosystem for all kinds of reasons.
Hell, they may just be grabbing some flowers and dirt for the missus when they get back home.
Yep - objecting to the ETH on the basis of alien motivations is perhaps the weakest argument imaginable, because any sentient being is going to have a range of motives and behaviors just as wide and varied as our own, if not more so. And we do all kinds of bizarre things for all kinds of bizarre reasons.
There are only two legitimate questions regarding the prospect of alien visitation:
1.) Are "they" out there? And based on what we know now from astronomy and astrobiology, the answer is almost certainly "yes, and they are legion: chances are that billions of advanced civilizations have arisen within the observable universe alone, and this is now a conclusion found in the peer-reviewed academic literature."
2.) Can they get probes or even occupied craft to the Earth? And based on our own technology, the answer to that question is "yes - we already have the capability of sending probes to nearby stars, and if they're sufficiently technologically advanced to have achieved 'applied general relativity,' then they could get here far faster than the speed of light so they could arrive from anyplace in this galaxy, or even other galaxies, and theoretically even from beyond the horizon of our observable universe."
All of the other arguments against the ETH, like "but they don't look weird enough" or "it makes no sense that they do this, that, or the other thing" or "there are too many reports of this, that, or the other behavior" (which is kind of an odd objection - that there's
too much evidence), are all empty objections given that we reside in an infinite universe with an equal range of possibilities.