why are the assignments, the actual routes that the gliders need to fly, not NOTAMed?
Because the route is not known in advance; at most the turning points will be pre-declared. In competitions the TPs are only decided on the day immediately before the competition starts - because they take account of the weather.
To see glider tracks, goto
Daily Scores then clicking on any of the "tasks" will popup a log of that flight showing track and altitude. One relevant example (both height and track) from Tibenham is
Flight Details