digital - What causes noise floor in an image sensor?

In the book Image Sensors and Signal Processing for Digital Still Cameras, it says that Read noise, or noise floor, is defined as noise that comes from the readout electronics.Noise generated in a detector is not included. In CCD image sensors, the noise floor is determined by the noise generated by the output amplifier, assuming that the charge transfer in the CCD shift registers is complete. In CMOS image sensors, the noise floor is determined by the noise generated by readout electronics, including the amplifier inside a pixel.

I remember that noise floor is dominated by dark electrons. What is the reason behind noise floor? In cameras, the output of sensors is subtracted by black level. So noise floor will not be one problem of sensor because it can be eliminated completely.

For the user of sensors, black level need to be subtracted. Why don't analog output subtract black analog value before ADC, so the digital output of sensors won't include noise floor.