>>27727966Because it's a video game.
Although if we apply some real-world knowledge to the situation, probably the same way most fish maintain their population. Gyarados is fearsome, and its biggest risk is probably being killed either fighting for territory with another Gyarados, or during mating should it get too rough. Still, they're fucking tough, and'll survive long enough to lay dozens of batches of eggs over their lifespan even with that in mind. Gyarados are (usually) salt-water Pokemon, but Magikarp is (usually) a fresh-water Pokemon, so we can assume a Gyarados will head up stream to mate/lay, much like salmon.
And those batches probably consist of hundreds of eggs. Most batches won't be preyed on, since no Pokemon'll risk going near a hormonal, territorial Gyarados, so they'll have a high rate of successful hatchings. The Magikarp will be hopeless, especially as young hatchlings, and the Gyarados will abandon them soon after they hatch, but there are so many of them that it guarantees at least a few of them will survive to adulthood/evolution.
And they evolve at lv20, which is a relatively low level for evolution in a two-stage Pokemon, so they probably don't take that long to mature into a Gyarados.
