Why do Ponzi schemes still exist if they’re scams? Why do MLMs still exist if they’re scams? Why does any scam exist if it’s a scam?
The answer is obvious: because there are still enough chumps to be tricked. There will always be people willing to believe anything, and there will always be others ready to exploit that.
Plus, you’re conflating things. Cryptocurrencies as technology aren’t inherently scams, it just so happens their features make them an extremely well suited tool to run scams with (which can take various forms), so much so that’s by far their most common use case.