When the truth was clear, they gave it a label.
Calling it magic so they could ignore it
Once Fir'awn felt safe and returned to his throne, he told his chiefs this was a well-versed sorcerer, and they agreed. Ibn Kathir says they plotted to portray Musa as a liar and a fake, to put out the light he brought.
A friend points out, kindly, that you snap at your kids when you are tired. You say: oh, she is just being dramatic. One word, and the point is gone. A label is a quick way to not listen.
If I can name what is wrong with the messenger, I can ignore the message.
Dismissing the speaker does not make the truth go away.
Recall one piece of advice you dismissed by labeling the person. Think about the advice itself for two minutes.