[bobjob]

The original question was "how come ghosts only talk to mediums?" and the obvious answer is "That's not actually the case."

Ghosts - more correctly 'spirits' - will talk to whomever they can reach in this dimension but mediums may provide the easiest way for some, perhaps many, to achieve communication.

If/when a spirit can reach someone directly then that's what she/he may do but the recipient of such communication may find it difficult to know how best to respond or, quite often, doubt they're understanding what's happening. Ideally they need support from someone experienced in these matters.