They cash it by something called money laundering. Let`s say I hack a exchange and get 7000 bitcoin, I could used a mixer and donate to myself on my stream or a friends stream, I could send it to 1000`s of streamers. Are you going to interrogate each one of them for receiving a donation? Now I the streamer cash out my btc into fiat or get someone else to do it on local bitcoins. They needed the btc because they see it is easier to steal than earn, the banks/governments do the same thing with interest and tax and laws, for example here in Canada only they are allowed to sell the booze and weed GANGSTER THUGS!
this is a streamer https://www.ccn.com/twitch-streamer-receives-a-donation-of-20-bitcoins-while-playing-runescape someone who plays games or makes videos on a live stream.
https://www.youtube.com/watch?v=qDzF7oAeaPA 1.7 btc donation
https://www.youtube.com/watch?v=SDUKB4NWMdM 4btc donation then like 4 and 4 and 4 $73K
https://www.youtube.com/watch?v=FmHAlUnfXRY my fav bitcoin alert
also those beggers you see on all the btc gambling sites and on twitter, they could be used as mixers