为什么 git 子模块在 git 签出时没有自动更新？

I believe that the submodules not updating automatically is in line with the development goals of Git. Git is meant to work in a distributed mode and doesn't presume that you are even able to connect to a non-local repository unless you explicitly tell it to. Git not auto-refreshing a submodule would be the expected behavior when thought of that way.

With that being said, if you know that you always want those sub-modules to be pulled in and you know that you would never branch off of those submodules to another local repository, then it shouldn't break anything if you automatically refreshed them after a checkout.

git checkout --recurse-submodules was added to git 2.13

This is mentioned on the release notes at: https://github.com/git/git/commit/e1104a5ee539408b81566066aaa6963cb87d5cd6#diff-c24776ff22455a30fbb78e378b7df0b0R139

submodule.recurse option was added to git 2.14

Set as:

git config --global submodule.recurse true

man git-config says:

Specifies if commands recurse into submodules by default. This applies to all commands that have a --recurse-submodules option. Defaults to false.

I feel that not updating modules by default is a bad Git default behavior that goes against most user's expectations and limits the adoption of submodules, I really wish the devs would change it.

With Git 2.27 (Q2 2020), the "--recurse-submodules" option is better documented.

See commit acbfae3, commit 4da9e99, commit d09bc51, commit b3cec57, commit dd0cb7d (06 Apr 2020) by Damien Robert (damiens-robert).
^{(Merged by Junio C Hamano -- gitster -- in commit cc908db, 28 Apr 2020)}

doc: --recurse-submodules mostly applies to active submodules

^{Signed-off-by: Damien Robert
Helped-by: Philippe Blain}

The documentation refers to "initialized" or "populated" submodules, to explain which submodules are affected by '--recurse-submodules', but the real terminology here is 'active' submodules. Update the documentation accordingly.

Some terminology:

• Active is defined in gitsubmodules(7), it only involves the configuration variables 'submodule.active', 'submodule.<name>.active' and 'submodule.<name>.url'.
  The function submodule.c::is_submodule_active checks that a submodule is active.
• Populated means that the submodule's working tree is present (and the gitfile correctly points to the submodule repository), i.e. either the superproject was cloned with --recurse-submodules, or the user ran git submodule update --init, or git submodule init [<path>] and git submodule update [<path>] separately which populated the submodule working tree.
  This does not involve the 3 configuration variables above.
• Initialized (at least in the context of the man pages involved in this patch) means both "populated" and "active" as defined above, i.e. what [git submodule update --init](https://git-scm.com/docs/git-submodule) does.

The --recurse-submodules option mostly affects active submodules.

An exception is git fetch where the option affects populated submodules.
As a consequence, in git pull --recurse-submodules the fetch affects populated submodules, but the resulting working tree update only affects active submodules.

In the documentation of git-pull, let's distinguish between the fetching part which affects populated submodules, and the updating of worktrees, which only affects active submodules.

With Git 2.33 (Q3 2021), the documentation for submodule.recurse is clearer:

See commit 878b399 (16 Jul 2021) by Philippe Blain (phil-blain).
^{(Merged by Junio C Hamano -- gitster -- in commit c018818, 02 Aug 2021)}

doc: clarify description of 'submodule.recurse'

^{Signed-off-by: Philippe Blain}

The doc for 'submodule.recurse' starts with "Specifies if commands recurse into submodles by default".
This is not exactly true of all commands that have a '--recurse-submodules' option.
For example, 'git pull --recurse-submodules'^(man) does not run 'git pull'^(man) in each submodule, but rather runs 'git submodule update --recursive'^(man) so that the submodule working trees after the pull matches the commits recorded in the superproject.

Clarify that by just saying that it enables '--recurse-submodules'.

Note that the way this setting interacts with 'fetch.recurseSubmodules' and 'push.recurseSubmodules', which can have other values than true or false, is already documented since 4da9e99 ("doc: be more precise on (fetch|push).recurseSubmodules", 2020-04-06, Git v2.27.0-rc0 -- merge listed in batch #4).

git config now includes in its man page:

A boolean indicating if commands should enable the --recurse-submodules option by default. Applies to all commands that support this option.