There are several possible reasons:
1. Maybe your kid wants to play without taking any breaks.
2. Maybe he or she likes the crinkling sound that they make.
3. Maybe he or she likes being wet.
4. Maybe he or she likes the designs on the front (if any).
5. Maybe he or she is jealous of the attention that a younger cousin or sibling is getting.
6. Maybe he or she is under a lot of stress right now and has no other way to relieve the stress.
7. Maybe he or she is having accidents (day and/or night) and doesn’t want to admit it.
8. It could be one or more of the above.
9. It might not be any of the above.
Asking him or her about it may clear up some of the answers (if he or she both knows them and is willing to talk about it).