I think it's quite common knowledge that diamonds were traditionally mined before we were able to make them in a lab, and because of this it's seen as the default to which any other methods of making a diamond are compared.
I guess I'm splitting hairs, yes there is definitely a cultural hangup about mined diamonds being "real" and made diamonds being imitations, fake, whatever. I just don't think many people care that much about the actual backstory, they just want a 'real' one. As such I think the synthetic diamond industry has a bit of an uphill struggle on its hands, but not an unwinnable one if they can convince people that theirs are 'real' too.
And to my (very literalist in many respects) mind, they are.
(edit - I will add that I have gone out of my way to source lab-created stones and silver settings to make jewellery for my partner in the past, so far mostly using star-sapphires, and she seems to have loved them. I find the idea of lab-recreated gems pretty fascinating, and I think that enthusiasm helped.)
They've been doing this to try and undercut lab diamond growing company's in an attempt to retain control of the diamond market in an era where lab diamonds are becoming more and more popular as people consider their ethical choices.