Why Z axis points up in the default scene???

shouldn´t it point to the interior of the monitor and the Y axis be the one pointing up?

“Which axis points up” is determined by the orientation of the viewport you’re “looking” through. Any of your modeling windows can be looking at from the front, from the side or top, or “through the camera.” (And you can freely alter the viewpoint of any of those windows.)

Obviously, in any coordinate space there’s no real concept of “up,” only that “the camera’s orientation is parallel to some particular axis of the object now being filmed.” As we see from time-to-time in carnival rides, the brain normally considers “top is ‘up’” but it constantly looks for reaffirming details in the background.

If you consider the NUM1 view front, NUM3 side, and NUM7 top, which is the most sensible way to think of them, then yes. Z is up. This is different from some other applications.

Why? It just is. As sundialsvc4 pointed out, it’s arbitrary, but if you’re used to something else you’ll have to get used to this in Blender. Maybe in the future the NUM viewport hotkeys will also be configurable, and it will be possible to set things up to have quick access to front, side, and top views with a different up axis.