Why do bash history files save timestamps in Unix (Epoch) time?

Issue

• When the HISTTIMEFORMAT environment variable is set in any way (for example: HISTTIMEFORMAT="[%F %T] "), bash stores timestamps for each history item and when a user inspects history with the BASH built-in history, the timestamps are shown in an easily-readable format (ex: 2013-01-29 11:40:19); however, when these history sessions are saved out to a file (e.g., ~/.bash_history), the timestamps are shown in Unix time (seconds since the Epoch). Why? How can these be read without using su to switch over to the user?