I cannot answer your question. I do not know why some do not believe in miracles.
I myself do believe. There are millions of Christians that do not believe in miracles.
Even if they themself got healed,they say that the body did this or something else.
If someone gets heals they say,they was not really sick. Yet they believe in Jesus.
I believe too rejoice, if not, religion has no purpose. Then we cant believe to anything. We believe in something more than us because we hope He will help us. No hope, no life!