Because her effect text simply says "banish 1 monster your opponent controls" not "target 1 monster your opponent controls, banish it". Learn the difference
If a card says "cannot be targeted by" then a card that says "target" in it's effect text can't do anything to it.
But if something says simply "Destroy 3 monsters on the field" then it doesn't target and isn't stopped by effects that prevent targeting. Does that help?
Yes I understand this but I still don't know what difference does it make for Every whether it says 'target one and banish' or 'banish one monster'. Isn't the effect the same?
From an outside perspective that do look the same, but no- they're not. I've already told you how target prevention works, but the other mechanic to it is if something "targets" it does so before the opponent makes a response- so it can be dodged easier (look at Dragon Spirit of White for a good example of how that works)
But non targeting effects need to be negated/dealt with before they go off, as they don't "target" in on something before that point- so they're harder to dodge
You still able to active eff, but right after you active and resolve it, the not targeting effect will take place afterwards. It wont have any relation with your prev. card, but instead the current condition.