In a general sense, Manual means manual. You do all of the work.
There are a few cameras that feature an "automatic manual" mode, in which you set the aperture and the shutter speed and the camera varies the ISO to suit. That's a new thing, and not quite the same thing as "full manual mode".
When you vary the exposure compensation on most cameras in manual mode, the only thing that changes is the meter indication. You have to change one of either the aperture, shutter speed or ISO yourself. You're in control, and you get to (or have to, if you find it a chore) decide which of the three elements controlling the exposure you want to adjust. Putting the camera in manual mode is telling it that you know what you're doing (which may or may not correspond at all to what the meter is reading). If you are using the camera's meter, then you can use the difference in reading between "normal" and "exposure compensated" readings to inform your decision.
But if the camera went ahead and overrode any of your settings without explicit permission, it wouldn't be manual mode, would it? How would it know which to adjust? If you were in aperture priority mode (A or Av), it knows that it's allowed to jigger with the shutter speed (until it hits any limit you may have set). If you're in shutter priority (S or Tv), it's allowed to vary the aperture (until it runs out of aperture). In "automatic manual" (each implementation has its own branded name), it's free to play with only the ISO. But in manual, what is it supposed to do?