If we have such a high field of vision, why can't we focus on everything within the vision simultaneously?

Why is it that I am not able to read a book without moving my eyes if the entirety of the page is within my field of view? Why do I have to center my eyes on an object to observe it fully? And why is it that I am still able to view changes in surroundings in the edges of my field of vision despite there being supposedly no way...