Because you need it to make purchases, just like a real world currency. When the Euro goes down, everyday people don't just liquidate all their holdings to USD, because USD isn't accepted in their countries. Take a look at dark.fail or the FBI statements, whenever they shutdown an onion market there is always hundreds of millions in BTC sales they uncover.

Without the dark web markets since The Silk Road in 2011, and now Tesla and big legal companies, coins would have no real purpose to exist. The hype alone isn't enough to keep them all this time and dark web markets aren't quick into adapting new coins each time. Many have ditched BitCoin to Monero only because of the privacy factor, not growth.