Why hasn't the CALDBVER keyword been updated after I run this tool?

The CALDBVER keyword isn't set by the CIAO tools, it is set in the pipeline. It is not, therefore, automatically updated by acis_process_events.

If you would like to manually change the value, e.g. to recall later what version was used in reprocessing the data, use dmhedit:

unix% dmhedit infile="acis_new_evt2.fits" filelist="" operation="add" \
      key="CALDBVER" value="4.5.1" datatype="indef" unit="" comment="CALDB version"

The Note on Processing Versions shows how to determine what version of the CALDB is installed on your system.

The chandra_repro script adds a comment into the output event header with the CALDB version it used. Users can use dmlist to list the value.

unix% dmlist acis_repro_evt2.fits header | grep 'chandra_repro was run' 
 --  COMMENT               chandra_repro was run with CALDB 4.5.1
 --  HISTORY               PARM  :value=chandra_repro was run with CALDB 4.5.1            ASC00639