Why do parasites survive in Nature? A huge bunch survive even though they have no or negative value to the host. They survive because that survival strategy, works for them.

Species emerge and survive where there is a unfilled niche in the environment. Bitcoin fills such niches the financial system doesn't serve. Such as black/gray markets, tax evaders, money launderers etc. All being as ancient as organized economies. When you ask why we haven't got rid of them, you start hitting deeper issues of human behavior, social and economic systems, that will take their own sweet time to deal with due to the level of complexity of the problems.

Other side to the story of why bitcoin survives is peoples reaction to loss of anonymity, bail outs of the "too big to fail" bunch etc.

Basically different groups are using the tool for their own reasons. Its pure luck they are in alignment. Which leads to more investments and innovation. If these groups find different tools to solve their problems then the story changes.