Those neutrons may cause other nuclei to undergo fission, leading to the possibility of a chain reaction.
The neutrons also slow due to elastic and inelastic collisions with fuel and other materials in the reactor.
Some will be lost purely due to geometry, those released travelling outward from the outer edge of the fuel mass will not have a chance to cause fission, for instance.
This is not simply an accounting of the total number of neutrons, as it also includes a weighting based on the energy.
Thus, remaining high-energy neutrons are not a major part of the "overall economy" as they do not maintain the chain reaction.