It all comes down to cost. These BGA chips have pins under the device unlike older packages like QFP or DIP. The pattern of missing pins is designed so it a fewer signal layers to bring out all the sigal connections in a typical PCB process. Devices with full grids would need more layers or tighter tolerances.

The appeal of STM32MP1 is you can put DDR momery with the chip on a 4 layer board. ST even provides the layout.