EV QUESTION
Because it doesn't make sense. The exposure compensation works my overexposing/underexposing the automatic controlled setting. Example :
In Av mode, you dial in +1 exposure compensation and the shutter speed will be one stop slower.
In Tv mode, you dial in +1 exposure compensation and the aperture will be one stop "more open"/brighter [i don't know the best way to say it]
In Manual mode you control everything, so what would the exposure compensation do? Nothing.
DOF and longer focal lengths
It does. If you get a 200mm 2.8 and frame a person, you'll get a nice defocus.
Here, for example :
That is a picture of my aunt using my Sigma 70-200 2.8 at longest focal length and 2.8 aperture.
If i had used a 24mm f2.8 and same framing, the background wouldn't be nearly as defocused.