Of course it's difficult. Even if you could convert it to cash you wouldn't be able to deposit in any bank or meaningfully use it. The moment you do anything with it you'll trigger anti money laundering laws and have to explain where the money came from.
From a criminal perspective you may not have to launder it. Just deposit your XMR/ZEC into an exchange and sell it. If they ask, say you bought it years ago at $10.
It's not actually that tricky - because obtaining Monero anonymously is also tricky, the only way to do so is to put on a pair of rubber gloves, and anonymously send cash from a mailbox that can't be traced to your identity. The other side of that transaction is the party interested in converting to cash. Because there's so much demand on both ends, there are many reputable vendors with a long and trusted history of those transactions.