How do people logically or scientifically claim that ghosts definitively "do not exist," given that there is no proof either way?

I completely understand the stance of "there is no evidence or impact, so it's practical to assume they don't exist." That makes total sense to me.

But what I don't get is how some people can flat-out assert or prove with 100% certainty that they absolutely do not exist. Since it's a "proving a negative" situation (the devil's proof) where there is no evidence for or against them, what kind of logic or scientific reasoning are they using to make such a definitive claim?

I'd love to hear your thoughts or the philosophical/scientific basis behind this kind of absolute denial.