Why do an adult housefly lays so many eggs at a time?
Not all egg-laying animals lay "many" eggs; some raptors and
penguins, for example, only raise one chick to adulthood at a
time.
Those that do lay many eggs have young that face high mortality
rates from predation, lack of food, or other factors. Only the
fittest (perhaps luckiest) survive to adulthood.