1. Because the math doesn't work.
2. Because light is radiation. Dimensions are something else entirely.
3. Because gravity is a force. Dimensions are something else entirely.
Dimensions are the set of coordinates it takes to define everything within something. Physicists are trying to define everything within the universe, beginning with our three dimensions we perceive and moving out to other physical dimensions. If the math works and they are right, then hopefully the standard model of physics will come out of the equations.
Time is also described as a dimension because it can also be a set of coordinates we are moving through.
Light and gravity just don't work that way, as far as I can tell. Of course, I'm no physicist. You might be on to something and I'm just totally missing it.