Why does "git pull" get all branches from repository but "git pull origin master" not do so?

The latter command, git pull origin master, tells git to fetch and merge specifically the master branch (from the remote named origin, to be even more precise).

git pull fetches updates for all local branches, which track remote branches, and then merges the current branch.

From the documentation of git pull:

git pull runs git fetch with the given parameters and calls git merge to merge the retrieved branch heads into the current branch

When you call git fetch without arguments, the following happens

Fetches named heads or tags from one or more other repositories, along with the objects necessary to complete them.
git fetch [fetches] from (...) a single named repository (...)

When you add arguments, only the specified remote and head (=branch/tag/commit/...) are fetched, and then merged.