Double deck vs wide body airliner, why would anyone build a double deck one?
Problems with just making the fuselages wider:

With the wing or main gear supporting the center of the fuselage, the front and back are cantilevered. Think of a hard landing or 2.5G pull up. The most effective structure to support bending is tall, with material on top and bottom (like an I-beam). A flat fuselage would have to be heavier to provide the same strength.

Depending on how you shape it, you're trading wasted space above and below the cabin for having an efficient pressure vessel shape. If you want to fit standard cargo containers below, that constrains your options.

Although you may get some amount of lift from the fuselage it will probably still push the wings and engines outwards by some amount, creating issues with gate width limitations.

In order to meet evacuation requirements, there would need to be more doors or at least emergency exits for a given length. That could mean more cross aisles and less room for seats. It may also be easier to access multiple doors for passenger to board/disembark if they are on different levels than the same level.

Lower ratio of windows to seats. Less natural light. People like window seats.