Just to expand on the other comments: yup, you're missing something! :D
Specifically, git is a DVCS/DSCM (distributed version control system/source code management system), which means that each "checkout" of the repository that exists is a complete clone of the original, including the full commit history and a cryptographically verifiable codebase.
In this world the "master" repository is simply a convention: everyone agrees that <insert person here> has the "master". But in reality everyone's copy is a peer of every other.
Sharing between repositories can be done any number of ways. Github is convenient, but originally email was the way code moved between copies. In fact, I host a number of my own private repositories over straight SSH.
So Github is a useful piece in the puzzle, but fortunately it's not a single point of failure in any way.
Specifically, git is a DVCS/DSCM (distributed version control system/source code management system), which means that each "checkout" of the repository that exists is a complete clone of the original, including the full commit history and a cryptographically verifiable codebase.
In this world the "master" repository is simply a convention: everyone agrees that <insert person here> has the "master". But in reality everyone's copy is a peer of every other.
Sharing between repositories can be done any number of ways. Github is convenient, but originally email was the way code moved between copies. In fact, I host a number of my own private repositories over straight SSH.
So Github is a useful piece in the puzzle, but fortunately it's not a single point of failure in any way.