Because the combustion chamber is occluded by compressor rotors and stators on one end and turbine rotors and stators on the other. And the outer shell of the engine is cold metal.
What you are seeing on the front is the fan blade, which really isn't anywhere near the core of the engine. It may be more open but there's still a lot of metal in the way... If you look through an engine from behind you can see light, but that's light shining past the fan and through the bypass route, not the engine core (where combustion occurs).