Because Jesus was baptized that way.
“As soon as Jesus was baptized, He went up out of the water.” — Matthew 3:16
Every baptism in the Bible was by immersion.
(Example) “…then both Philip and the man went down into the water and Philip baptized him. When they came up out of the water…”
Acts 8:38-39
The word aptize means “To dip under water” The Greek word baptize means “to immerse or dip under water”. It is best symbolized by a burial and resurrection!