Ahem, to amplify just a little: to do that, you'd have to breach a hole in, or break outright, the wards of Waterdeep. Picture a pancake (the surface city) balanced on a grille of three widely-spaced bars (the wards), atop a pit (Undermountain). To pluck a dime-shaped piece out..

...of the pancake, using tweezers or tongs, and pull it away (removing the Yawning Portal), is going to tug on the pancake. With a high chance of it falling between the bars of the grille, down into the pit. Which is why the wards are designed to resist any such endeavour.