1) Because exactly this happens

2) Because developers check out that source code and store it on their own laptops, which can sometimes get lost, stolen, or hacked.

3) Because with a proper secrets manager, each developer can get their own auth credentials for each service, increasing auditability.

Ask Mercedes-Benz. If somehow that repo gets exposed - so are all of your secrets. You just made a problem a lot worse and potentially opened up an avenue to expose more data.