Except Chelsea actually tried to hold Courtois against his will..

He told Chelsea he wanted to leave, they agreed, then he had an amazing WC and Chelsea pulled back. Then the whole shit show, where he didn't show up to train and forced a move, happened. But is not uncommon at all that players who don't have as much leverage as him (young, probably top 5 in his position, after a great run in the world cup, on the last year of his contract), are not released by the club and just never get played.

Just look at Rabiot.. He is not even training with PSG's A team, but he has a contract so unless some big club splashed a lot of money to get him in the last window (like the player wanted), PSG could just keep him there and make him run his contract down (like they did)