Here's why:
* Power Tool Dragon's effect is a Continuous effect. It permanently increases the ATK of any machine-type monster you control by 500. It doesn't directly target or interact with other monsters' effects.
* Destroying a monster does not automatically negate its effects. The only time a monster's effect is negated by destroying it is if the effect itself specifically states that.
Example: If a monster has a Continuous effect that boosts its own ATK, destroying that monster will simply remove it from the field, not negate its previous ATK boost.
In summary: Power Tool Dragon's effect is a permanent boost, and destroying it doesn't negate its previous effect.