Why isn't the cosmic microwave background a solid 3D volume?

This is the cosmic microwave background radiation:

It's a Mollweide Projection, which maps the surface of a sphere. Like, this is a map of Earth (also Mollweide Projection):

And it only shows the outer surface of Earth. You can't see the inside.

What I don't get is, how can the cosmic microwave background radiation be mapped as a flat sphere? The CMB is everywhere in the universe, so why isn't it a solid 3D volume? How come it doesn't look like this:

Why is the CMB a sphere as opposed to a volume?