Because the light that was created on day one, is different than the lights (plural) created on day four...sun stars, etc.
Now, what that light was on day one, I don't know, and I don't pretend to know. All I know is that there is a distinction between the two. This failure, by many, to understand this very clear, simple point, leads me to believe that those who raise an argument such as yours, simply have not read the text with any type of diligence.
That said, let me refer you to one of your previous posts in which you propose that creation is from God's perspective. In fact, I agree with you, so why is it so hard to understand that God would establish such a standard unit of time, from the beginning, for the purposes of understanding and communication with his special creation...and by extension, he would set the earth in motion to comply with this standard.
What is so hard to understand about that.
Day and night only require a source of light. Read Rev 21:23.
The day was 24 hours long.
You are correct that your confusion is not of God.