There are three reasons:
Biblically: Baptism by immersion is what we see in the New Testament. The word “baptize” is an old word that actually means “to completely sink under water.”
Theologically: Going under the water represents your choice to die to your self. Coming up out of the water represents Jesus raising you up to walk in new life.
Symbolically: It’s helpful to think about baptism as a visual way of depicting what has already happened spiritually. Your old self is gone. You’ve been raised with Christ!