Because the survival rate is low. Not many of them will make it to adulthood.
There’s a concept called [r-K selection](https://en.wikipedia.org/wiki/R/K_selection_theory) in biology with regard to reproductive strategies. Basically, it’s about whether a species puts a lot of resources into a few offspring, or few resources into a LOT of offspring. Elephants and humans are on one extreme, turtles and a lot of fish and insects are on the other.
* Elephants will gestate a single calf for 2 years, and it spends its whole childhood and adolescence being cared for and reared by the mother and the rest of the herd.
* Human children spend 18 years, give or take, under their parents’ care.
* Trout eggs get fertilized in a streambed and they’re on their own from there.
Crocodilians are somewhere in the middle. They’ll build a temperature-controlled nest, and guard it, and provide a limited amount of care when the babies are quite small, but after that, they’re on their own. A lot of them get eaten by predators (raptors, herons, raccoons, otters, some kinds of fish, snapping turtles…).
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