Here's why:
* Flip Summoning: Flip Summoning is a specific summoning method where a face-down Defense Position monster is flipped face-up and its effect is activated.
* Setting a Monster: When a monster is set, it is placed face-down on the field in Defense Position.
* Timing: For a monster to be Flip Summoned, it must have been set during a previous turn. The turn it was set, it cannot be flipped.
Example:
If you set a monster during your turn, you cannot immediately Flip Summon it during the same turn. You'll have to wait until your next turn to be able to Flip Summon it.
Note: Some monsters have effects that allow them to be Flip Summoned even if they were set during the same turn. These effects are specific to individual cards and will be clearly stated in their card text.