Because newborn elephant calves need to be large.
A newborn elephant calf weighs about 100 kilograms. Developing such a large organism from a single cell takes a considerable amount of time—especially considering that the grass and shoots that elephants eat contain few proteins and other necessary "building materials" for the growing body.
For this reason, the gestation period for elephants lasts up to 22 months (whereas, for example, the blue whale, which feeds on krill, has a gestation period of no more than a year).
It is not advantageous for elephants to give birth to smaller and less developed calves. Firstly, the calf must be able to follow its mother and the herd within a few hours after birth. Secondly, its size provides protection from most predators. Moreover, pregnancy does not significantly reduce the ability of the elephant cow to find food.
