Why must motion be with respect to the oberver?

Answer 1

That simply means that there is NO WAY to define or measure an "absolute motion".Any experiment you do will be the SAME for different observers - in the sense that it is unaffected by relative velocities.

Answer 2

Motion is relative, meaning the perception of motion depends on the observer's frame of reference. When we describe motion as being "with respect to the observer," we are acknowledging that an observer's position and motion can affect how they perceive the motion of other objects. This is encapsulated in Galileo's principle of relativity, which states that the laws of physics are the same for all observers in uniform motion.

Answer 3

It need not be. With respect to you, as the observer, the sun, moon and stars are moving around the earth. I hope you do not still believe in the geocentric universe!