Why hasn’t this been done already?

First, it is way harder to do.Every imperfection introduces secondary effects.

Second, the people that make those diamonds are not ashamed or hiding but believe on what they do. Leo Di Caprio for example invested millions in making synthetic diamonds that do not finance wars in Africa.

New generations are proud of people not dying for getting their beautiful stones.

lastly, diamonds, like gold, are very useful for science. In particular they are amazing semiconductors with incredible heat dissipation and small power loses. If we get better, less defects diamonds ,cheaper, power electronics will be revolutionised.