They have. They’ve been able to do this for many years now.
The problem is artificial GEM grade diamonds. Diamonds, regular diamonds, are common as hell. They’re used in industrial applications all the time.

It’s that gem diamonds are more rare and cartels like DeBeers have tricked millions with a slick marketing campaign into thinking they’re somehow valuable. They artificially inflate the prices by releasing only a few onto the market every year while sitting on warehouses full of them.
Why else do you think that if you buy a diamond the moment you buy it the value goes down? It’s artificially kept high. And they’re terrified of synthetic gem grade diamonds that could compete with their business.
So what are they doing now? Selling the trash diamonds as “fancy” diamonds.
Back in in the day there were three colors that were considered good. Clear, blue and pink. Yellow diamonds were considered trash and worthless. Now? They’re marketed as ‘Canary Diamonds”. Brown diamonds were unwanted. Now they’re “Chocolate Diamonds”.
Diamonds and gems are some of the biggest scams out there. They are not worth the money DeBeers places on them.
The first artificial diamond was synthesized in 1955. You can now buy artificial diamonds for jewelry and sensor windows alike. There are a number of techniques utilized, which are discussed in the above link. Besides giant industrial diamonds, synthetic diamonds can also match natural gem-quality diamonds .
Here’s a 3.5-inch wide artificial diamond:

And this company manufactures large artificial diamonds for laser and sensor windows.

Ye olde high pressure/high temperature technique for synthesizing diamonds can create all sorts of gem-grade diamonds (these are awaiting polishing):

(Almost) gem-quality chemical vapor deposition diamond:

Polished high pressure/high temperature synthetic diamonds:

