Some children chew on their shirt because they are bored or anxious, nervous etc and other children chew on their shirt because they need oral stimulation.
I used to chew on my shirt as a child when I was bored or anxious so in my case that was the reason I chewed on my shirt.
I used to chew on my shirt sleeves as well.
There was nothing wrong with me but some kids may do the same for other reasons such as being sensory sensitive or even when they have autism.
Some shy or embarrassed children also chew on their shirts as a way to calm themselves down.
It's usually nothing to worry about and eventually the child will stop chewing on their shirt.
You can try giving them some fruit to chew on and eat instead of chewing on their shirt.