Why do birds only lay a few eggs?

Eagles are large birds and their young when they hatch out of eggs are pretty big when compared to the young of other birds. Caring for too many kids (Esp with the amount of food they eat) may get difficult for the eagle parents which will bring down the chances of their survival. Hence 1-3 eggs in one brood is easier to manage and make sure the kids survive.

This may be the reason why evolution ensured that eagles have only around 3 eggs per lot.