There is a rumour that the high doses are because the correct dose was under patent.

I can only find that in a comment here: https://slatestarcodex.com/2018/07/10/melatonin-much-more-th... but I could have sworn it was in the main article when I first read it. (If so that makes it less likely, because the author revises his articles for correctness)

That (albeit long) article is worth reading. For example, it explains that melatonin is a phase signal, so being saturated with too much melatonin actually makes the signal less clear (as well as potentially causing other issues).