Inherited from old times and to not break stuff. Every line in
/etc/passwd needs a home (see the addition below). And
/etc/passwd is something we take as is as part of the Linux system (and not a Debian/Ubuntu specific feature).
In the old days syslog-ng was pretty common and that used
/home/syslog/ to create a directory in there for each data source type.
Before the switch to
systemd using rsyslog, storing logs in
/var/log/syslog was more common. And systemd uses
See the man page:
/etc/passwd contains one line for each user account,
with seven fields delimited by colons (“:”). These fields are:
· login name
· optional encrypted password
· numerical user ID
· numerical group ID
· user name or comment field
· user home directory
· optional user command interpreter
The encrypted password and user command interpreter explicitly are mentioned as "optional". So I would assume the others are mandatory.