Of course the cross and the crucifiction of Jesus is a major symbol of Christianity.
You also asked why people don't use other symbols like "outstretched hands -loaves and fishes -Jesus getting baptized -Jesus laying a healing hand on a sinner," and I replied that these other symbols are often used too.
Along with the Virgin Mary (which I'm sure you have seen many times)in many forms.
And also Annunciation to the Blessed Virgin Mary, when the angel Gabriel comes to the Virgin Mary to announce that she is going to give birth to the son through the Holy Spirit.
Christianity is not lacking in variety or volume of religious symbols.