Master can also mean proficiency. If you say you’ve mastered a trade it doesn’t mean you enslaved the trade, you simply have complete knowledge of the trade.

So in that context, the master branch is the complete branch. The branch that other branches stem from because it’s the one with code from all the teams. You could branch from another team member’s branch but if that branch hasn’t merged from master in a while, it won’t have all the knowledge (code). When you merge in master you’re getting knowledge from elsewhere from the branch that’s aware of more things than your branch is: the branch that has mastery of the code, the master branch.