You added some keys temporarily, and before you know it one of your teammates has pushed the change to github.
Or you kept your certificates on your internal file server, and someone recycled the hard drives without wiping.
Or you shared your connection strings with your team mates by secure mail. But one of them lost their phone in a taxi cab.
Or you decide to be cautious and decide to only keep in your head. But then how do you deploy automatically every few hours?
As your team grows 10x, rate of mistakes grows 10x.