People buy inferior, counterfeit merchandise all the time because they can't tell the difference.
But there is nothing inferior or counterfeit about a manmade diamond. It is *exactly* the same material as a natural one.
Yes, you can buy a man-made diamond the same you can buy counterfeit branded clothes. That only shows that you’re tasteless and that you can’t afford the real thing.
If it is strictly about the money, a larger man made diamond can easily cost just as much as a smaller natural one --- while instantly conveying to the untrained eye the appearance of being more expensive.
If you see someone wearing a 6-carat diamond, you know it's either a $100,000 natural diamond, or a $4,000 lab-grown diamond. And you'll assume it's the latter, as far more people can afford the latter.
On the other hand, a 1-carat diamond would be more like $2000 natural, $300 lab-grown, making it far more believable that it was natural.