At one point the Defense Professor does give a plausible reason why he might have resolved not to use Legilimency.
From Chapter 74:
His head fell back against the chair, lolled to one side, the eyes no longer meeting Harry’s. “But these small games hardly hold my interest as they stand. Add Legilimency, and it ceases to be a game at all.”
At one point the Defense Professor does give a plausible reason why he might have resolved not to use Legilimency.
From Chapter 74: