So the pdga is more worried people getting next to a car, next to a bug/animal(that will move), or people getting their feet wet in casual water than falling into a river or getting a rash that sometimes involves dr visits?
I really think a rivers edge is sometimes the most dangerous part of a course. That needs to be looked at.
They can't write rules to legislate river's edges. That's for the TD or course designer to account for. If it is truly dangerous, then the TD or course designer should be the one to address it. The rules of play are written with the assumption that the course being played has been vetted for safety concerns before it is used.
Plants don't cause rashes in everyone. A lot of people couldn't identify poison ivy if they wanted to, even when they are susceptible to allergic reactions. Everyone can identify a car without question...or an insect/animal...or water. Allowing relief from plants would require all players to be able to readily identify it otherwise what's stopping a player from saying any random plant is dangerous just to be able to take enough relief to get a better angle around an obstacle?
The rules do account for any individual player being able to avoid situations that they find dangerous. If for any reason a player wants to avoid a particular position, they have the choice to re-throw from the previous lie with a penalty or to take line of play relief with a penalty. Players can elect to do that at any time on any lie.