Arguably, there are more such crystals than those which have been made. It is just that nobody bothered to write them all down, because hey, what's the point.
A tiny minority of those theoretical crystals haven't been made for the reasons you envisioned, that is, because the conditions of crystallization are hard to achieve. Here by "conditions" we mean pressure because temperatures that are relevant to solid state chemistry can't be much of a problem. Slowly and painstakingly, one by one, these crystals are being dragged into existence. Oganov's discoveries are relatively recent examples. Metallic hydrogen is still waiting for its turn.
A vast majority, however, haven't been made for an entirely different reason: not because the conditions are hard to reach, but because there are no such conditions. If you want to make them, you have to invent some ingenious workaround ways.
You surely heard of graphite and diamond, and of the amount of effort it takes to turn the former into the latter, and that it's been successfully done. Now, what about this guy?
Diamond does have a place (albeit far away on the phase diagram) where it is stable. Fullerenes don't. I can claim with a good deal of confidence that you won't find the conditions where they are thermodynamically stable, not even in the areas where we haven't reached yet. However, they have been made. Others are still waiting:
(more can be found at https://en.wikipedia.org/wiki/Allotropes_of_carbon)
So it goes.