In absence of good, hard, scientific evidence of effects above and beyond placebo, I’m gonna go with “Cos it’s easy to bottle and it’s easy to sell”.

And profitable too, I have no doubt. Thanks to Orrin Hatch, US supplement peddlers don’t even have to prove their products are safe, never mind effective:

https://en.wikipedia.org/wiki/Dietary_Supplement_Health_and_...

I bet the oft-reviled Big Pharma (who, amusingly enough, are some of the biggest manufactures of supplements too) just wish all their products could enjoy such lax standards of regulation. Meanwhile, to everyone else, I wholeheartedly recommend The Poison Squad:

https://www.pbs.org/wgbh/americanexperience/films/poison-squ...

https://www.penguinrandomhouse.com/books/312067/the-poison-s...