Why do we experience ontological otherness to self?

Why do we experience ontological otherness to self?

Why is physical reality not-self?

Why is matter a negation of selfness?

Even our own bodies are not truly self, but only infused by self—if you chop off my arm, I am still me. The voice that I take to be me—the one that utters “I”—is unscathed even after the loss of a limb. After all, I will be the one complaining about it—proving that the speaker has not been divided.

How can there be areas of non-self?

This question is what drove Fichte and Shelling to their early deaths.