It is not as ridiculous as you think. ACH transfers can be canceled and reversed. Copinbase waits for 5 days in order to reduce the risk of fraud.

Try this: open an account at any bank (other than your current bank) and transfer money from you current account to this new account. Time how long it takes before you are able to withdraw cash from your new account. I bet it will be no faster than Coinbase.

Most people are ignorant about how money and banking work. One of the benefits of the Bitcoin movement is that people are starting to understand what money is and realize how incredibly inefficient the banking system is.

For example, a few years ago someone in the US banking industry came up with the crazy idea of ACH being done instantly rather than taking 3-5 business days. Of course, the idea was considered absurd even though it is a trivial to implement. So, the industry instead voted on reducing the time down to a single day. The banks voted overwhelmingly against it, so we are still stuck at 3-5 business days.