In kinetic theory we always try to derive equations that relate macroscopically averaged properties of the system. This means we only consider quantities that are averaged over distances much greater than the average distance $d$ between the atoms, and times much longer than the average time $\tau$ between collisions. This is because these averages are all we can measure, given that our measuring devices (thermometers, pressure gauges etc.) are also macroscopically large.
If you could actually build a "pressure gauge" that measured the force acting over an area $d^2$, with the ability to respond to changes over times shorter than $\tau$, then your gauge would register a wildly fluctuating value. At this point the very concept of the pressure stops being useful, because you can resolve what the individual atoms are doing. But normally your pressure gauge measures the force over an area that is huge compared to $d^2$, and responds much more slowly than $\tau$. Therefore the effect of these fluctuations cancels out and you only see the average. So the derivations you are looking at are actually only working out the average force over some area and time that are much larger than $d$ and $\tau$.