thermodynamics - Why no need to consider collision in kinetic theory?

In standard elementary thermodynamics textbook, in the kinetic theory it is assumed that the gas particles have no interactions except during collisions. However, in the subsequent derivation of

$$pV=\frac{2}{3}U$$ only the collisions of the gas particles with the container walls are considered, but not the collisions between gas particles. Why we can ignore it?