I've never understood why "high" became the expression for intoxication because (at least with drink) it was more a case of struggling to remain upright rather than any sense of being lifted up.
"Stoned" is equally incomprehensible unless it refers to achieving a state of insensibility?
The trouble is, we mostly just copy the slang we encounter. Questioning what every piece of slang means just makes you look ignorant and 'uncool'. I suspect only the originator could say why they chose such contradictory terms, to describe their experiences.
But, as the saying goes, if you can remember the sixties, you obviously weren't there. If you asked them, they probably wouldn't be able to recall.