Only because they're made by Sony haha
Not quite true anymore. The 24 MP sensor in the D3200 is a Nikon design. More bizarrely, the 24 MP sensor found in the D5200 and the D7100 is a Toshiba sensor, although it is not clear whether they are building a Nikon-designed sensor, or whether it is their own design (more likely, though).
Lastly, the 16 MP sensor in the D4 is made by Aptina, if I recall correctly. Not sure who builds the D600 sensor.
So from the current range, only the D800 (and maybe D600) has a Sony sensor.
Still, it should not really matter who manufactures the sensor, unless you want to fuel a brand war