Light is not instant because it isn't is about all we can say. Physics tries to describe the universe, it doesn't necessarily try to explain why things are that way. As an example, Newton set out to produce a working theory of how things move and especially how they are affected by gravity. His theory appears to allow gravity to act instantly at any distance, something he acknowledged as being doubtful. Newton's get out was to "frame no hypotheses". In other words, he didn't attempt to explain the workings of gravity, he just devised a description which worked well enough to make useful predictions.
Now to deal with your specific case. We are travelling at a substantial fraction of the speed of light. How do you know? What did you use as a reference to measure that speed? That's the thing about relativity, it says there are no absolute fixed reference points. If we measure our motion relative to a star for example, how do we know it is not the star that is moving? It could be that both of us are moving. We have no way measure the movement absolutely. What we do know is that it doesn't matter whether we are moving, our reference point is moving, or even which direction we are moving, light will still be measured as having the same speed no matter which direction it comes from. Relativity just sets out to describe this mind boggling state of affairs.
The theory says the closer you are to the speed of light the more time slows down the why isn't light instant? we all know that light can go only so fast bu because of the theory why isn't it instant please someone explain it to me...