Because they're sold as supplements and not as medications. Many supplement dosages are arbitrary, with few exceptions.

The same author posted a follow-up post explaining why: http://slatestarcodex.com/2018/07/12/did-a-melatonin-patent-...

In summary: "Commenters, including a patent lawyer, have filled in the rest of the story. Because melatonin is a natural hormone and not an invention, patents can only cover specific uses of it. The MIT patent covered the proper way to use it for sleep; a broader patent might not have been granted. The patent probably guided supplement companies, but expired about five years ago. It’s now legal to produce melatonin 0.3 mg pills, but people are so used to higher doses that few people do."

It is not their motivation but what's great about those huge doses is that you can cut the pills and the bottle of pill last two or four time as long.

Europe. I get mine melatonin tablets there. Just stock up for a year. Small bottles to bring back. Never had an issue through customs.

I suspect it's just marketing. Consumers like seeing the big numbers because they assume bigger number more effective.