I'd go with the "firing of synapses without purpose" hypothesis. It if was actually productive, evolution probably would have made it available to us without drugs. Psychedelics are not special, these are relatively simple molecules imitating neurotransmitters. So if tripping were so beneficial, it could probably be triggered through normal pathways, with the added bonus of being able to switch from high to baseline at will.
Until the very recent (in evolutionary terms) formation of large social groups that could support the development of technology, a state of hyperactive brain activiy that consumes even more resources than the brain already does, would not have been something that was very beneficial and selected for thru evolution