Why is telepathy considered a delusion and a sign of psychosis? explanations please?

A delusion is a: false belief caused by a mental illness and can be a sign of psychosis. The belief in telepathy in one didn't before, who suffers from mental illness and outside of a certain cultural belief systems, could be considered a delusion. WIthout signs of specific mental illness, believing in telepathy by itself is not a delusion nor is this psychosis. Many folks + cultures feel this way.