I dig my own trails but I'm one of the lucky ones who has found somewhere to do it that doesn't get flattened. I get irritated by the predictable comment of 'Go build them then' because for the vast majority of people it really isn't that simple.
its not simple for 99% of the trail builders out there. You work your arse off, you get into legal wrangles with councils/landowners, you finally get them on your side only to find some drunk f*ck has put a big hole in each of your takeoffs cos he can't stand other ppl having fun in ways he can't manage. I'm sure its the same the world over. You just have to knuckle down and get on with it. There is no simple answer...
When you find out that your trail and all your time and effort have been destroyed by some stupid f*ckhead, it's the worst feeling concerning mountainbiking (except injury). It happened to me also this winter. Many hours and weeks of work were destroyed just because some f*ck thought that the part of the forest my trail was going through belonged to his family. I had only built my trail in this spot because I was told that the property belonged to my uncle. So basically after some legal actions, it finally turned out that the f*ckhead mismeasured the borders of his land by nearly a quarter mile. So now I'm back digging every weekend to recover at least some parts of my trail