because she cries dramatically when she falls over.
On the other hand if DS who has just turned 12 falls over, cuts himself/grazes he gets up and deals with it. I have no problem with him going to the park without an adult.
DD however lies on the floor, cries dramatically, and shouts that she's hurt, and sometimes even screams she can't get up because she can move her ankle/wrist/whatever, even though it's probably just a little graze she's suffered.
DH has let her go to the park with DS this week, but I'm not convinced she's mature enough.