Probably because they think that most of the hard work that goes into building a browser -- like parsing sloppy html, handling CSS correctly, a JS engine, etc -

Probably because they think that most of the hard work that goes into building a browser -- like parsing sloppy html, handling CSS correctly, a JS engine, etc -- were not done by the author. To me this feels like like getting a free cheeseburger, adding a leaf of spinach to it, and then saying you built a cheeseburger. But hey, maybe you want a cheeseburger with spinach, and you don't care whether other people think they "built" the cheeseburger or not.