To clarify the question a little; are you asking why space and time are reffered to as a package together (spacetime), or why both space and time are spoken of as if they come in little packages (quantised)?
Maxwell created an open wound in the world of Physics when he proved that the speed of light was independent of the motion of the observer. So all things material still obeyed the Galleilan velocity transform laws (essentially, velocities add) while electrical phenomena did not. This wound remained open for nearly 50yrs until Einstein's paper on relativity was published. A.E. used Maxwells constant speed of light to derive Lorentzes emperical electrical velocity transform equations. To accomplish this he linked space and time, showing that that they are intimitly tied together.
It is not clear what you mean by "magnitudes sort of related" Space and time are different axis of a coordinate system related by c, the speed of light.
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