Well it does because he was the guy who thought it up. LSD was in existence for a decade plus and no-one ever mentioned "ego-death". There's not many of Leary's ideas that are still thought of as being true.
Who cares that he made up the phrase "ego-death". He didn't make up the experience. When I first had this experience, I had no idea there was such a phrase as "ego-death", but when I read people talking about it and describing the experiences they had, I thought 'wow, huh I know what they're talking about.' I mean, this is not complicated.
Besides, the general metaphor of "death" in the context of dramatic shifts in self-understanding obviously predates Leary.
Well as I've already explained - taking alcohol makes you lose all sense of who you are. I guess you'd call that an "ego-death" whereas I'd call it "being drunk".
Yes, and falling asleep also makes you lose all sense of who you are, but that doesn't mean that falling asleep, getting drunk, and tripping on psychedelics all affect the sense of self in the same way. These are completely different mental states, subserved by completely different brain processes.
Like, when Dwayne says, "I had remembered that I was really some meta-being whose thoughts had generated the entire universe," I know exactly what he means, because I've had this experience myself. Other people have also had it. No one ever has an experience of this class when they drink alcohol. I have had experiences where I felt that my self was co-extensive with everything around me, and that I was making the Earth spin and the Sun shine. Do you ever get really drunk and feel like objects external to your body are parts of your self? No? What a surprise.
Now on the other hand, we know that there are many other examples where the sense of self is distorted in similar ways. For example, paranoid schizophrenics who experience delusions of thought insertion do not recognize their own thought processes as belonging to themselves. People with alien hand syndrome do not recognize the affected hand as belonging to themselves. There is something called the rubber hand illusion, where the subject watches a dummy hand in front of him being stroked as his own hand is synchronously stroked out of his line of sight. This subject experiences the dummy hand as part of himself even though he clearly sees that it is unattached to his body.
What does all this mean? Well, obviously it means that "the sense of self" is something made up by the brain. Something that can be manipulated and distorted, shrunk, expanded, and altered in various other ways. Psychedelic drugs can do this. In fact, they can do this so dramatically and so effectively that as it is happening, you may feel that you are dying. Which is not surprising, since your sense of self is what you think you are.