How come boys are more athletic?

Boys are not necessarily more athletic, but boys tend to be stronger and more enduring in physical activities than the average girl - this is a simple matter of Biology. Males are typically larger, have greater muscle mass, and possess greater upper-body strength than females of a comparable age.

Males are also driven by fluctuating levels of testosterone during puberty, making them more competitive. It is a plausible theory than males become more competitive and females more nurturing (generally) during puberty as an evolutionary requirement.

However, athleticism has more to do with attitude and motivation in adulthood. A trained female will tend to be considerably more athletic than an untrained male (and vice-versa).