Here's why:
* Detaching materials is a cost to activate its effect, not a protection mechanism. Slacker Magician's effect only activates when it is targeted for an attack by a monster. Detaching a material allows you to banish a card from your hand, but this does not negate the attack.
* The attack still goes through. Even after detaching a material and banishing a card, the attacking monster will still attack Slacker Magician, potentially causing damage.
Therefore, detaching material on Slacker Magician does not prevent it from being attacked.
It's important to remember that Slacker Magician's effect is a reactive one. It can only be used when it is targeted for an attack, and it does not prevent the attack from happening. It simply allows you to banish a card from your hand.