Here's why:
* Summoning: When you summon a monster, it is either summoned in attack position or defense position.
* Attack Position: The monster is face-up and can attack directly.
* Defense Position: The monster is face-down and can only defend.
* Changing Positions: You can only change a monster's position after it is already on the field.
* Attack Position to Defense Position: You can change a monster from attack position to defense position during your Main Phase 1.
* Defense Position to Attack Position: You can change a monster from defense position to attack position during your Main Phase 1 or Battle Phase.
Therefore, you cannot summon a monster in attack position and then immediately change it to defense position. You have to choose its position at the time of summoning.