This is something that's actually studied in cognitive science, you're free to watch the (very long) lecture series if you're interested, the transformative experiences being discussed are very much not about "some weed smokers."

The series is pretty clear about the actual contents of altered states being largely garbage, so your observation and skepticism regarding "amazing ideas which they somehow can never write down and keep for later" is exactly in line with some of what's being discussed there, not contrary to it.