I'd see 2 reasons:

1. A lot of people put a lot of money into it and they won't give them up. They'll keep buying and selling, keeping it sort of artificially afloat even if it has no real world usage. Well there is actually one which leads me to the next point
2. The illegal market (and gambling) has a use case for cryptocurrencies so they actually use them

But to put it simply - they don't die because they don't have to. There is no single company that would pull the plug. By it's design, they can coexist in our world and no one can stop it, doesn't matter if people use it or not

It's like a torrent with millions of seeders. As long as there is at least one seeder, the torrent will exist even if the files it contains aren't really useful