Low stakes with string or wire strung among'em should be enough in most cases to deter off-path tromping eh ? Plant something inside their perimeter so it all looks preplanned.
It's generally desired to put up as few artificial features as possible to maintain the natural look of the park. Sometimes it is necessary, of course.
The other problem is that many trails are just maintained by a couple of volunteers. Many in the US are done by local mountain biking clubs, for example. It can be hard enough to maintain what is already there when you're relying on the goodwill of a couple volunteers to do heavy labor at 6am on a Sunday.
That 20% there is the little fact that all the fences and signs in the world don't keep a certain subset of headstrong people from e.g. leaving the path and falling into geysers in Yellowstone, or getting too close to wildlife.
