Why does Judaism encourage questioning but frown upon “idolatry”?

(Ok so don’t think in trying to slander Judaism here or anything like that, just a simple question.)

I think most of you know that “Judaism encourages questioning” is something you’ve heard before. Yeah that’s great, obviously some begin to question Judaism itself. For some, they’ll start doubting the validity of mitzvot, the existence of God, and other things.

Sure, there are atheist Jews, and treated well among Jewish groups. And not all Mitzvot are followed by everybody.

But what happens when you question Monotheism and possibly leaving for other Gods, and then act on it? Probably, some people will call you an Idolater.

But why? You can question Adonai’s existence but not the possible existence of other Gods? Why are Jews that embrace and worship 2 Gods (Wicca), 3 Gods (Christianity), or more Gods (Shinto, Hellenism, etc) sometimes considered traitors, apostates, or Idolaters, when they are doing what Judaism encourages: questioning?