Why do kids not like to zip their jackets

Marion1985 thought this was worth mentioning said

My little sisters are 5 and 7 years old and will come out of school with their jackets wide open but, I know that is because both prefer big sister to do theirs up.

I have noticed when I am picking up my sisters that all kids at their school, aged between 5 and 10 will come out with their jackets undone too, even during the cold/freezing winter months so I’m just wondering, is there any particular reason(s) why kids will prefer not to do theit jackets up or get parents/guardians to do it for them?