I agree with Rockrose. Many kids find this way instinctively to focas.
I would also add that my son does it to soothe himself when he is a bit nervous. It can show that a child is feeling anxious too. Our occupational therapist has told us that many children try to soothe themselves through this type of chewing or sucking.
It is best to find alternatives to allow him to still self soothe vs trying to stop him. They make chew things at various occupational therapy websites, you can also string some beads and he can wear them around his neck and suck/chew as needed. A great thing for while at school are coffee stirrers.
But truth be told, my son still will go for that sleeve even with the alternatives . . .
To bring his brain into focus.
Children who chew their shirts typically do it during reading or math. The act of chewing or gumming something actually focuses the brain.