To know why time is not absolute, we must date back in time and have to see the evolution of theories regarding space and time. At first, Aristotle proposed that the natural state of a body is to be at rest and starts moving only when it gets acted upon by some external force. It followed that a heavy body should fall faster than a lighter one on to the Earth. Later it was proved wrong by Galileo,and Newton proposed his three laws on the basis of Galileo's experiments. He proposed that, the natural state of a body is not to be at rest but to be in uniform motion and only when it gets acted upon by some external force, It starts accelerating. It follows from Newton's laws that there is no unique standard of rest. Lets for instance, a train was moving past an electric pole standing tall along the embankment, at a speed of 70 mph, one could equally say that the pole was at rest and the train was in motion or the train was at rest and the pole was moving past at the rate 70 mph; we can't really say which one is at rest preferably. If for someone inside a train which is in motion, an object appears at rest, and it appears in motion for someone outside the train. This implies, we can't assign any event an absolute position in space. hence space is not absolute.
They came up with non absolute space but still it was believed that 'time' was completely independent from 'space' and that one can unambiguously measure the interval between any two events that happen in space. In 1676, Christensen Roemer came up with a significant discovery that light travels at a finite speed and he calculates it to be around 140,000 miles per second, However later Clerk Maxwell precisely measured it to be 186,000 miles per second. Whatever it is, It adds a significant implication that light travels at a finite and more importantly fixed speed.
Here comes the most interesting part, if light has to travel at some fixed speed, It must travel relative to something at rest and what could be that 'something'?
They came up with a shockingly interesting and imaginary space, 'Ether', supposedly presents everywhere and obviously, at rest.
Later scientists tried to measure the time taken for the light (from a fixed source) to reach an object that is moving towards it, in one instance and moving away from it in another instance. Shockingly, they measured the same 'time' in both instances.
At that stage Albert Einstein came up with his famous Theory of relativity, which implies that time is not absolute, obviously no need of ether anymore. A remarkable consequence of relativity is the way it has revolutionized our ideas of space and time. In Newton's theory, if a pulse of light is sent from one place to another, different observers would agree on the time that the journey took (since time is absolute), but will not always agree on how far the light traveled (since space is not absolute), Which implies that different observers would measure different speeds of light. In relativity, on the other hand, all observers must agree on how fast light travels. They still, however, do not agree on the distance the light has traveled (since no absolute space), so they must now also disagree over the time it has taken. In other words, the theory of relativity put an end to the idea of absolute time, which means that time measures between any two events in space by two identical clocks would not necessarily agree, even though they show the same.
We must accept that time is not completely independent from space, but is combined with it to form an object called space-time