Why doesn't an animal question it's sanity?

Because that would interfere with it's survival.

And the same is true of most humans.

When people question their sanity it is mostly about feeling that they "Don't fit in" with one group or another. It's not really that most people are not sane it's only about fitting into a group.

For example, when a religious leaning person is born into an atheist family this could be a problem and the same thing could be true of an atheist leaning person being born into an either religious family or a family that is spiritual but not religious here in the U.S.

So, often it is about contrasts between different people or a single person contrasting with a group. It doesn't necessarily mean that the group is wrong or that the individual is wrong necessarily.

After all there are many kinds of people in the world.

Religions traditionally might consider anyone different than them at core as "possessed" but that usually isn't true in the end. Mostly it is people moving to the beat of a different drum than them.

I have found that most fundamentalist people (whether they are Christian or any other religion) have been victimized usually as children and so become intolerant of anything "different" in any way in other people.

But usually that does not mean that those "Other" people are "possessed" or have a "Demon". It is quite possible that fundamentalist religious people would consider every wild animal on earth to actually be a demon too, especially if it is a wild tiger and comes and eats one of their children.

But, is that wild tiger possessed or a Demon really? No!

It's just an animal doing what comes naturally.

What are humans at core?

We are all animals who have been domesticated through religions and cultures (which are basically the same thing) if you study anthropology, philosophy, sociology, psychology etc.

And all religions originally evolved up from Shamanism. Even Jesus was a Shaman in that he fasted for 40 days and nights. The same with Buddha who also fasted and met God and his own demons this way. Only Buddha realized that Compassion was the key rather than making a big deal about whether God existed or not. So, there was just a difference in emphasis here compared to most other religions on earth.

But, Culture always comes from Religions. And Religions all started with Shamanism and evolved up from there. So, understanding this is helpful when we realize we are all just animals who were domesticated by our religions which we now call cultures.