Because an evil user can maliciously try to point the file
root is writing to a different location.
This is not so simple, but really possible.
As an example, if a user would find the way to make a symlink from the supposed Apache log to, say, /etc/shadow you'll suddenly have an unusable system. Apache (
root) would overwrite your users' credentials making the system faulty.
ln -s /etc/shadow /home/eviluser/access.log
If the access.log file is not writable by the user it can be difficult to hijack it, but avoiding the possibility is better!
A possibility could be to use logrotate to do the job, creating the link to a file not already existing, but that logrotate will overwrite as soon as the logs grows:
ln -s /etc/shadow /home/eviluser/access.log.1
The symlink method is only one of the possible attacks, given as a proof of concept.
Security has to be made with a White List mind, not blacklisting what we know to be an issue.