Because the act of thinking is a mild form of hallucination. (Hearing a voice in your head that no one else can?) Dreaming is another example. "Using your imagination" is another.
Anyone can hallucinate, and under the right conditions, can become dlelusional. >>1650
Nothing like you're describing, the most I've experienced sober is hearing my name called out when no one's there, and intense math models I've tried to work through visually.