Most people who have spent the time and effort to produce a presentation are reluctant to simply give it away to someone else.
Secondly, as someone who has run selection boards it was quite common for candidates to give a brief put together by someone else(usually commercial) and then fail to understand it. We recognised them because we had seen them before.
If its worth giving a brief, then its worth putting it together yourself, that way you should understand it and often in the process of doing it you learn something you didn't know.