They get opportunities in practice. Thats where playing time is earned. Malachi came in and instantly earned a starting job cause he was crushing it in practice. Same with Fitzgerald. Some guys take a few months of practice to show it and some never beat anyone else out. I would assume Carr is trying hard in practice but just not outshining players ahead of him. Blocking could be an issue as well. Not something I have heard, but it's always possible.
Good luck to him elsewhere. Hope he picks a good fit for his game and does well.