Creating tons of coinbase accounts is no easy task. You have to validate the accounts with multiple forms of ID and this pattern would eventually be found out and get flagged big time.
Then, you need different bank accounts for every single coinbase account you create and those require multiple forms of ID as well as valid SSN numbers which will be checked.
If you are referring to GDAX allowing you to transfer to USD, looks like it is FDIC insured, which means it is a US bank account, which means it is subject to KYC patriot act stuff and will be the same as opening any other us bank account.
You do this so that each individual account can make small purchases over a long time horizon so it looks valid. Looking at coinbase's site, you only need a bank account if you're using fiat currencies. You'd probably also want to make it harder to detect all your laundering accounts by having them make purchases to other "legitimate" accounts too (e.g overstock) Obviously I'm not a criminal mastermind so this plan could very well fail, but I find it hard to believe that a digital cash currency couldn't be used to effectively launder money considering laundering is most effective for all-cash businesses (combined with good accountants/bankers like illegal organization employ these days to avoid all these problems).
Otherwise, you have to have a seemingly legitimate Enterprise that is accepting Bitcoin on a large scale. Which would cause you to stand out like a sore thumb to the authorities because no one in their right mind is using Bitcoin with $40 transaction fees.