Here's why:
* Summoning Mechanics: Monsters are summoned either face-up in attack position (Normal Summon, Tribute Summon, Fusion Summon, Synchro Summon, Xyz Summon, etc.) or face-down in defense position (Set Summon).
* Face-Down Position: Face-down defense position is only used for setting monsters. This means they are placed on the field face-down and can't be used for attacks or activated effects.
* Face-Up Attack Position: The only way to summon a monster face-up is in attack position, which makes them available for attacks.
Important Note: While you can't summon face-up in defense position, you can flip summon a face-down defense position monster to turn it face-up and change its battle position. This can happen when a card effect or a specific card's effect lets you change a monster's battle position.
Let me know if you have any other questions about Yu-Gi-Oh! summoning mechanics!