Here's why:
* Flip Effects are triggered by flipping the monster face-up. This is an independent action from summoning the monster.
* Summoning a monster doesn't automatically activate its Flip Effect. It only sets the stage for the possibility of activating the effect later.
* To activate a Flip Effect, you must specifically flip the monster face-up. You can do this by:
* Attacking with it: Flipping it face-up during the Battle Phase.
* Activating a card's effect: Some cards allow you to flip a monster face-up as a cost or effect.
In summary, the act of summoning a monster doesn't negate its Flip Effect. It just means the effect is available to be activated later.