I agree that it should have been noticed earlier from a few vantage points. I'm managing over a dozen contractors which varies from dev to marketing to customer support, operations, sales, financials, investor relations, etc. Everything could be better, but it's a business and the squeaky wheel gets the oil. I never fathomed the squeaky wheel was going to be AWS usage charges.
I myself have never created an instance, I set up the account then gave access to the devs. I only log into the account to provide new access to devs that need it and none of them are full time.
Ultimately the responsibility lies with me, but I would disagree that my dev team is clueless. Rather they're working on development, not watching what the servers are doing on a daily basis so I think that's a bit unfair.