Why did the soldiers cast lots for Jesus' tunic instead of tearing it?

Answer: It was seamless

The tunic was seamless, woven in one piece, so they cast lots for it, fulfilling "they cast lots for My clothing."

“When the soldiers had crucified Jesus, they divided His garments into four parts, one for each soldier, with the tunic remaining. It was seamless, woven in one piece from top to bottom.”— John 19:23 (BSB)