If the existence of ghosts were somehow proven scientifically and it was shown that ghosts can effect things in the material world then logically ghosts could also effect the outcome of scientific experiments. That means if ghosts are real then a ghost in a laboratory could invalidate the results of any experiment. If you couldn't trust the results of an experiment then that could very well be the end of the scientific method. The end of the scientific method would mean the end of science as we know it.
Now you know why scientists so want you to believe there are no such things as ghosts.