It’s fun to think up thought experiments that explain away the reasons we can’t prove they exist.
Time isn’t linear, we’re just limited to experiencing it that way while we’re alive.
After we die, we experience time differently; in an impossible-to-describe way, but is akin to a book with limitless pages. Your existence is a bookmark in this book, and dying makes you lose your place.
It is possible to re-find your place, but with unfathomable access to unfathomable histories and futures, there is a near-infinite choice of other events and timelines for a ghost to visit and observe and experience.
Coupled with that, there are laws— some sort of physical laws, not arbitrary— to visiting the pages of this book, in that they can only be done in areas that do not cause pages before or after to change dramatically. A ghost is a spec of ink on the page; it cannot write letters, words, or sentences. If anything like this were ever done, we would never know as our minds would simply accept the memory as fact without knowing, or delete the memory of a ghost the way we space out driving on a highway or having to look at our watch a second time.
Lastly, our senses are all on varying spectrums; some of us able to see, hear, smell, experience things others cannot. One of our senses is a sense of time. Some people’s sense of time operates on a scope of a wider caliber than others, experiencing things others cannot, which is why some of us may have experienced ghosts while others haven’t.
edit: I guess it needs clarification based on the downvotes, I don’t believe any of what I just wrote (except the true bits like time not being linear & sense scopes). I don’t believe in ghosts/have never seen one, but I try to stay open-minded for friends who might disagree.