More likely: "no one has any idea what these old credentials do, so let's not touch them and potentially break everything"
I'd definitely consider a "silent" credential - a credential not registered centrally - to be a huge red flag. Either it could get stolen, or break and no one knows how to regenerate it. And it's pretty easy as devs to quickly generate an auth key that ends up being used permanently, without any documentation.
Sure, but you aren't going to do all that when your team is juggling N other priorities. At least, it will be very difficult getting mgmt and others on board. Unless it's explicitly in the context of a recent breach.