Because it ruins the Mystery, There is always the chance we will see the Time War but it seems to work better with us not seeing it.
Many of the details of the war are like that. The name Nightmare Child, for example, suggests something really horrible. Everyone knows it was horrible. If it were shown, no matter how good the effects, there would be some people who didn't find what they saw especially bad. There might be others who found it seriously and lastingly disturbing. The name on its own does the job without risking disappointment or damage.
Plotwise, the more we know about the war, the more it's likely to conflict with what scriptwriters want to do.