Sure, but as a public company they're legally responsible for the audit they signed off and while the Big 4 refuse to work with private crypto companies, Deloitte did choose to audit Coinbase. I just don't believe they can get away with this level of shady stuff while a Big 4 audit them and they're under extreme scrutiny for the sector from the SEC.
Before it was the "Big Four" it was the "Big Five". Look into what happened to that fifth one... (Arthur Andersen re:Enron)
It's worth noting that while we got the Sarbanes-Oxley Act out of it, it was more or less replaced by Dodd-Frank Act, which was partially repealed by Trump in 2018.