It’s one single place where you can search for most everything you could want, while also being only one place to hand out your monetary info and address to. It also heavily favors buyers (like ebay), so if there is an issue, it’ much more of a sure thing to get a no hassle refund. Entering your credit card into dozens of differently designed websites the owner of whom may or may not know how to even spell “security” and stores everything in plaintext, or just outright may be a scammer, requires more mental overhead and risk.

Now, in reality amazon is basically an internet flea market, and those supplements you bought are probably either a fake knockoff of a brand and made with lead, or “genuine” but falsely reporting it’s contents with no oversight and also filled with lead, but it’s the idea of buying them through “a familiar and therefore trustworthy place” that matters.

Also the 2 day “free” shipping with prime really brought a lot of people onboard.