In order to see something with your eyes, it needs to first have matter. Ghosts don't have matter so how can we see them?

The
theory is that when someone sees a ghost, it's telepathy. The ghost
consciousness projects itself to a person and then their brain creates
the image of a ghost. The reason ghosts wear clothes is because we're
seeing them the way they see themselves. When you close your eyes and
picture yourself, don't you imagine yourself wearing clothes? Also,
most people when they picture themselves don't see themselves right
down to their feet, which could be why ghosts are often seen without
feet. (Auerbach)

Why do ghosts appear in pictures? That's
Psychokinesis (PK), or "mind over matter". We know from experiments
that PK affects film, so that's what the ghost uses to appear in the
picture. "In their lab the Soviet scientists [Sergeyev and Ivanenko]
had observed that while making objects move, [the subject] Kulagina
caused exposurelike effects on sealed photographic film. Moved objects
actually left distinct traces across the sealed film." (Broughton,
p.142) Sometimes the ghost may only project itself to one thing at a
time, which is why we may get something in a photo that we didn't see
with our eyes. However, just because you didn't see it with your eyes
when you were taking the picture doesn't make it paranormal. It could
be a camera defect. It could be dust illuminated by the camera flash.
It could be the camera strap or a finger in front of the camera, etc,
etc.

Electronic Voice Phenomena (EVPs), or ghost voices caught
on tape, are PK too. The ghost doesn't have a mouth or vocal cords to
make speech, so it has to manipulate the voice recorder using PK. The
reason ghosts are able to "hear" our EVP questions when they don't have
ears is telepathy. But you don't die and just suddenly you are able to
do all these things. They have to be learned, like Patrick Swayze in
the movie "Ghost". That could take forever, which is why older
hauntings are sometimes more active. (Auerbach) Some may never learn
how to do it, or it might be really hard, which is why we can't sit
down with a voice recorder and have a whole conversation with a ghost.
They might not even be able to hear us.

References:

Classes on Parapsychology taken with Loyd Auerbach

Broughton, Richard. "Parapsychology: The Controversial Science"