How can anyone say for sure that spirits DON'T exist? ?

If someone asks whether or not I believe in ghosts/spirits, I say yes and explain why. I respect the opinions of others and realise that usually, unless you've actually had an experience, you would naturally dismiss the idea that they do exist. I lived most of my life not really believing, I didn't give the whole "spirit" thing much consideration. Without going into detail, an experience I had proved without doubt that there is more to this world than just what we see through our eyes; whether it be the energy left behind etc, why do some people flat out refuse to consider it? I realise that there's often no concrete proof, so that makes me feel like maybe they aren't "meant" to be seen. Why some of us have seen them and others go through their entire life never seeing one is a mystery. For the record, I'm a perfectly sane adult, a mother and am not "one of those" people who go around looking for spooks; I don't believe in vampires, fairies or unicorns, lol. I just know what I saw some 15 years ago.