I read a Fortean article years ago when I bought a magazine suggesting black cats emerging from a portal,but what’s the reasoning behind it?
Absence of evidence that would normally be expected to be present is a form of evidence.
With so many reports of ABCs, you might reasonably expect some sightings to be supported by strong evidence such as footprints, DNA, good clear photos or video. Generally (with a few exceptions) this evidence is absent.
So how do we explain the numerous sightings but the lack of evidence? One solution is to postulate an interdimensional portal such as might be consistent with string theory, dark matter, and the many universes hypothesis. Or, to put it more simply, use a lot of big science-derived words to say, "I made up an explanation."
Part of this is the desire to have one simple consistent explanation for a phenomenon. However, in real life, it is entirely possible that there are numerous explanations for different examples of what superficially appear to be the same phenomenon. Reported sightings may be hoaxes, pareidolia, escaped big cats, hallucination, mistaken identity, and so on.
Personally, I do not find the absence of evidence as compelling as some do. I do a lot of driving at twilight or at night, and occasionally I cycle, walk, or run in the twilight or dark. I often see muntjac, foxes, and badgers, and occasionally larger species of deer. These sightings typically last under 10 seconds, and it would be almost impossible to get a decent photo of them.
Years ago, I used to visit Dartmoor regularly. The place is no doubt teaming with foxes, badgers, deer, stoats, weasels, adders, grass snakes... but I never saw any of these species out on the moor. I very occasionally saw them crossing the road, or on the verge just as they disappeared from view.
I think I'm pretty observant. As a motorcyclist, I often find myself pointing out buzzards, red kites, kestrels, unusual vehicles, buildings or signs to my wife who rides pillion.
However, as an insurance investigator, I used to interview people who had been involved in collisions who had never seen the other vehicle until the moment of impact. Most people go through life in a little bubble, barely noticing what is around them when it's important, never mind when it doesn't affect them.
There is the famous video (find it on you tube) where a man in a gorilla suit walks across in front of the camera, and most viewers do not even notice it because they are focussing on what they are "meant to be watching".
It would not surprise me if ABCs were "seen but not observed" on many occasions. The evidence of their existence may easily be concealed from detection in thickets and undergrowth off the beaten path. I have seen roe deer on the road within 500 metres of my house, but I have no idea where they hide the rest of the time. There are many small patches of unmanaged woodland that I seldom visit and which most people never even try to visit.