Everyone's clock is different (time relativity). Everyone's ruler (distance relativity) is different. We can't be on the same page when all the pages are different sized and shown at different times.
Simultaneity makes sense in a world that only has Newtonian mechanics. Meaning, all rulers and clocks always agree on observed events....but that's not the world we live in. We live in a relativistic one.
Imagine an observer that is standing still and an observer that is moving at constant velocity. Lets say the observer that is standing still records all events "perfectly". If you want to ask the question: "what would the guy moving at a constant velocity see?", that question is answered by doing a Lorentz transformation from the point of view of the guy standing "still" to the view of the guy who is "moving".
In Newtonian mechanics, such a transformation is merely just a +C (change in physical position). Lorentz transformation, on the other hand, isn't that simple. It has a V/C2 in it....
The end result of that is that when you transform the view from one observer to the next, you end up with a bunch of observers who can't agree on the sequence of events.
It's not a question of how long light takes to reach your eyes or some other simple explanation. It's really fundamental to the universe to say that Simultaneity is just a thing.