The reason we have cash is (1) we used to not have widespread electronic instant settlement systems and (2) because you still generally need some type of physical fallback system because we energy sources and communications networks aren't always up.
Bitcoin is bad at 1 - 7 transactions per second (something something lightning network), and completely fails at (2) because it can't settle transactions without the internet, and electricity.
That may be what you think of as the core reason for cash to exist, but you shouldn't speak for others, who may see the privacy as a more relevant upside.
Cash enables some illegal activity, but at least for me the benefits FAR outweight the obvious consequences