Abraham is 99 years old: and has got a promise from God.
“If you'll be My friend and walk with Me, I will multiply you greatly.”
But, Sarah sees the issue. She sees the reality.
Now Abraham and Sarah were old, well advanced in age; and Sarah had passed the age of childbearing. Therefore Sarah laughed within herself, saying, “After I have grown old, shall I have pleasure, my lord being old also?”
This prompts an all-knowing God to ask, "Why did Sarah laugh, saying, ‘Shall I surely bear a child, since I am old?"
When God asks why: there's a bigger purpose.