https://support.coinbase.com/customer/portal/articles/1662379-how-is-coinbase-insured-
that's not entirely true.
if you read closely, coinbase obtained insurance for its online crypto storage. according to them, that means "less than 2% of customer funds". so let's say there was an inside job that compromised the cold storage. the theft of cold storage funds would not be covered!
they do have FDIC insurance for USD deposits and possibly equivalent insurance for other fiat currencies. but the bulk of their crypto storage is not insured. it would probably be prohibitively expensive to ensure all their customer deposits.