Why is sendmail is part of the system and not a package?

[b. f.]

>It's in /usr/sbin/sendmail.
>
>How many people actually use it? Very few.
>Why isn't it moved to ports?

Obviously, not everyone wants or needs sendmail in the base system.
But quite a few people do use it, and many FreeBSD developers are
happy with the status quo, so it is unlikely that sendmail will be
removed soon.  But there's nothing to prevent you from disabling it in
rc.conf(5):

sendmail_enable="NONE"

and replacing it's administrative use with local logs in
periodic.conf(5), by adding, for example:

daily_output="/var/log/daily.log"
daily_clean_hoststat_enable="NO"
daily_status_mailq_enable="NO"
daily_status_include_submit_mailq="NO"
daily_status_mail_rejects_enable="NO"
daily_queuerun_enable="NO"
daily_submit_queuerun="NO"
daily_status_security_output="/var/log/daily.log"
weekly_output="/var/log/weekly.log"
monthly_output="/var/log/monthly.log"

. (Or you can use another MTA instead.)  You can also go one step
farther: if you have the system sources available, you can rip
sendmail out of the base system and avoid building and installing it
again by using either

WITHOUT_SENDMAIL=yes

or

WITHOUT_MAIL=yes

in src.conf(5), then running 'make delete-old' and 'make
delete-old-libs' in /usr/src, and finally removing any leftover
associated files by hand. (find(1) can be used with  the appropriate
flags to check for stale files or empty directories in the base system
directories immediately after a fresh install in order to help locate
such leftover files.  A warning: use of the more drastic WITHOUT_MAIL
option can remove /usr/bin/fmt, which is used unconditionally by some
src targets.  So you may need to install fmt by hand, or patch the src
Makefiles so that fmt isn't used.)  All this doesn't take very long,
and doesn't need to be done all that often on an existing system, so
the presence of sendmail in the base system shouldn't worry you too
much, even if you don't want to use it on your system(s).

b.