Likely because they require complex structures processes to create, not just pressure and time and heat. "Danger to others" is not a factor in nature's selection process.
Danger to others that share a gene line IS part of natural selection. That's why most creatures don't cannibalize within their family/pack for instance.
That's not what the OP was talking about however. The fact that carbon fibers are not present in nature only tells us that we didn't have to evolve anything protecting us from it if it's really dangerous (since it was never a selection criteria). That doesn't mean it's necessarily harmful though.
Disagree; OP was talking about why no animal in nature produces carbon fibre, given its dangerous to others nearby. Was it ecological pressure? I answered, yes that's possible.
Carbon fibers are pretty complex to produce, that alone can explain why they're not naturally present in nature. It would be similar to wondering why silicon microprocessors are not naturally synthesized by nature.
It doesn't necessarily mean that evolution ruled it out, just that it may never have considered it in the first place because it never came to exist under the stochastic process of evolution.
He was contrasting it with diamond and graphite. I don't think animals produce those either. It seems more like the topic was why nature in general does not produce carbon fiber, not animals specifically.
