because when you spoof whatever is fingerprinting you sees that you're actually providing fake data, so now you enter the list of the "hidden" instead of blending in. this is the core philosophy of tor browser, and a known fact. if you read partially the article i linked it talks about this.

You can lie to the website and say your resolution is 1024x768. But what happens when the JavaScript fingerprinting checks the actual width of the view port? Your view port is most likely larger than your stated screen size.