The parents may be the same. But the genes may be different. Genes play an important role in the growth of the child. Almost all will have two copies of most of their genes. These copies will be different. We pass one of our two copies of each of our genes to our kids. Which copy a child gets is totally random. This is the main reason.
But the title of the thread suggests something else. It raises the question of why siblings born to the same parents differ in characteristics, habits, education, intelligence, wisdom, knowledge, emotions and abilities etc.
What the scientists say about it is that it all depends on genes. As they say, we share half the genes of our father and half of our mother and these genes are made up of DNA molecules that determine how each part of the body are to be developed. And changing just one gene in a sequence of genes can change the result utterly. Therefore, two brothers/sisters may share half of their genes but it is possible that they don't share some of the sequences of genes that are responsible for intricate facets of their personalities.
Knowledge is power.