Why doesn't the `time` command work with any option?

Well, even if you don't like it, I will put you to read again with more attention man time. At the end of EXAMPLES section you will find:

  Users of the bash shell need to use an explicit path in order to run
  the external time command and not the shell builtin variant.  On system
  where time is installed in /usr/bin, the first example would become
       /usr/bin/time wc /etc/hosts

So, I'm assuming that you use bash shell which uses an internal version of time, provided as a shell keyword. You can check this using the following command:

type time

and the output will probably be:

time is a shell keyword

If this is the case, then is clear, to use the real time command, you must to use its explicit path: /usr/bin/time.

Further, if you don't want to use anymore the shell keyword time, you can create a permanent alias as follow:

alias time='/usr/bin/time'

This will overwrite the shell keyword time because the command:

type time

will give the following output now:

time is aliased to `/usr/bin/time'

Since, as the other answers explain, time is a shell keyword, the only option available to you is -p:

terdon@oregano ~ $ help time
time: time [-p] pipeline
    Report time consumed by pipeline's execution.

Execute PIPELINE and print a summary of the real time, user CPU time,
and system CPU time spent executing PIPELINE when it terminates.

Options:
  -p    print the timing summary in the portable Posix format

So, you need to run the time that's in /usr/bin. Here are a few ways to do so:

• Use the time executable instead:

  /usr/bin/time -f %Uuser ls >/dev/null
  

• Use \ which causes your shell to ignore aliases and keywords and instead search your $PATH for a matching executable:

  \time -f %Uuser ls >/dev/null 
  

• Use the command builtin which has a similar effect:

  command time -f %Uuser ls >/dev/null
  

• Use a different shell, one that has no such keyword. For example sh (which is actually dash on Ubuntu):

  sh -c "time -f %Uuser ls >/dev/null"
  

• Use which, which will search through your $PATH (OK, this one is silly):

  $(which time) -f %Uuser ls >/dev/null
  

The bash and zsh shells have their internal time command. You have to use

/usr/bin/time -f ...

BTW, I discovered that using (from zsh):

~% which  time
time: shell reserved word