They're not though, because they are actually a different thing (plastic) and you can tell that from some distance.

Lab-grown diamonds are the same thing as natural, the same material, and can be produced to a higher specification for the same cost as finding (due to scarcity, labour, etc.) an objectively worse stone. (Or equivalently a given spec can be manufactured cheaper than it can be found.)

The correct flower analogy would be growing nicely coloured varieties in a nursery vs. hunting for those random mutations in the wild.

(Nobody does the latter!)