They used it because it was an established term
My graph theory is a bit fuzzy but I think that the definition of a branch in a directed graph corresponds to the path between two nodes/vertices. This means that by definition any path from the root node to any vertex is itself a branch.
I don't think Git invented this concept, nor did any other version control system.
I know that “branch” helps intuitively and visually when it’s actually an offshoot with one root and a dangling tip, like an actual tree branch…
I think that your personal definition of a branch doesn't correspond to what graph theory calls a branch. Anyone please correct me if I'm wrong.
That would defeat the goal of making it backwards-compatible with C.