Because you know as well as I do that he's not going to get in the team. He'll be 22 by the time next season starts and the highest he has played is 10 games in League Two. From contracted players for next season, he has Ballard, Alderete, Mukiele, Hume, Reinildo, Seelt, O'Nien, Hjelde and Alese ahead of him. So he's something like our 10th choice defender.
Using a club option to extend his contract (which will be peanuts) to try and generate some cash (or PSR headroom) is not good for him at this stage of his career - it jeopardises a crucial year for him, it may stop him joining a club he wants to join, it reduces his choice and probably reduces his wages because he's not a free agent.
It would be a shitty thing to do.