Think of it this way; there is less information in a blurred image (less colour, less lines, less areas). You can not* conjure information out of thin air thus making the unblurred image.

* Recent advances in AI actually make this possible to an extend. The AI delves into it's massive memory and extrapolates a likely image/face.

That entirely depends on how the image is blurred. The default gaussian blur in an image editing tool can be reversed without leaning on magic AI to do it.

Not completely mistaken, I think you're confusing blurring with "blending", where pixels are displaced in tight irregular spirals. These images have been successfully unscrambled as part of criminal investigations into child exploitation cases.