The way I see it, storing passwords in Git (or other version control) is a **convention**. I suppose one could decide not to enforce it with various results, but here's why this is generally frowned upon:

 1. Git makes it painful to remove passwords from source code history, which might give people a false that the password was already removed in current version,
 2. By putting the password in source control, you basically decide to share the password with anyone who has access to the repository, including future users. This complicates establishing roles within a developer team, which might have different privileges,
 3. Source control software tend to get pretty complicated, especially "all-in-one" systems. This means that there's risk this system might eventually get compromised, leading to password leakage.