'My point here: why are the assignments, the actual routes that the gliders need to fly, not NOTAMed?'
Gliders don't 'fly routes', they 'proceed in a general direction', which could be any direction at any given time........I've flown cross countries where I've been 10-15 miles off track desperately scratching for some lift..........and unless you're somewhere like Aboyne where people go for the wave up to 25,000 ft and more then expect gliders up to 10,000 ft at least.