Space is a substantial limiting factor. For us, we have to maintain a buffer zone with the shoreline which makes the spaces even smaller. When you crash on the ice and 60-70 mph you don't stop, you slide quite a way. Shore is an ugly thing to hit.
As to why the trench at Luderitz is so straight I can't say. That sweet spot, the balance point between crosswind and downwind, is the fastest moment in a run. You get into it, sheet in for a little extra pull and try to maintain the balance point as long as possible until you blow out and start going down wind. Maybe it doesn't work that way on water? (can't imagine why) Maybe they don't look for that magic point. Maybe they don't notice it or use it and the trench cut reflects this? I think I know someone who can give us an answer on that.