I find it strange that so many people are 100% certain they don't exist.
Exactly. The most intelligent, scientific attitude to it is that there is a phenomenon, cultural, social, physical, sociological, that we call ghosts.
My mum just bleats on (for attention) 'But ghosts can't exist'.
Well, why can't they? There are plenty of things we can't explain on a physical level.
My dad IS a physicist. He experienced a 'haunting' in my mum's old house. He has done a great deal of reading of quantum based theory about how particles interact and behave with each - not conclusively. If anything, firmly establishing that since there are particles that behave and interact in ways we can't understand or measure, then it's impossible to disprove one way or the other.
You can only perform a meaningful experiment if you are able to reproduce it - if there are factors that you can't measure or reproduce, then you can't know if you even did the same experiment twice.
(one of my dad's interpretations - and please don't grill me on this, because it's hard enough to keep up with his theories - is that particles can somehow take on a pattern of a person, and repeat that, in a way that the human brain can 'see' even though there's nothing there any more)
Long story short... 'Ghosts don't exist' is a highly unsophisticated scientific opinion!