Because we are born prematurely. Our brain wouldn’t fit through the birth canal if we were born later with more basic skills baked in. So instead we finish a big part of our early development outside the womb.

Point is that modern child rearing isn't even similar to what we had 3-4 generations ago. The "weak" were weeded out. If you had to choose between the family surviving and dealing with a finicky newborn... compromises were made.