A few things that cross my mind, but let me state at the outset I am a big Steve Cook fan and wish he was still a prominent figure in our ranks.
1. We don’t know if Cook had an option on his contract that meant if he paid x amount of games this season, it grants him an option of x years for a new contract on wages somewhat relative to his premier league contract (and would also be entitled as one of our higher earners to higher performance related bonuses regardless of achieving promotion) - therefore makes sense to let him leave because despite the player he is, we don’t want to be tied in to those commitments .
2. The board in cost cutting mode may have said “we can’t afford Cookie to play x amount of games and trigger a guaranteed contract for next season on x amount” let’s try to move him on, work with what we have and add Cahill on a 1 year deal (not sure if Cahills basic is less than Cookie but I assume so and with an option to renew that is lower cost)
3. Maybe we aren’t seeing Cahill because he’s getting close to an appearance based contract renewal we don’t want to trigger and instead chose to offset some of that potential future expense on Phillips to the end of the season