Because we did it like that in 1978?
Personally, I think we should fly true tracks all the time, none of this heading rubbish (magnetic, especially) now we all have GPS.
That aside, we have the ITTs and distances on our plogs, which are not subject to so much variation (ha!) as the magnetic info. We also have backup manuals containing this information and it is in true. I think producing all this paperwork keeps somebody in a job, which is probably the real reason why we do it...