This is a method to add trees and landscaping.

> Once a forest is added to an area, it becomes completely inaccessible because of how dense it grows. All of the ones I have seen are completely fenced off.

It sounds like they're completely inaccessible because of fencing and density. Fencing might be hard to remove, but density at ground level tends to decrease as a forest grows; the canopy blocks sun and lower tree limbs may drop or become less productive and bushier plants have a harder time. After the forest gets somewhat established, pathways are easier to form (although brambles and thickets may need help to clear), and thinning the trees can help the remaining trees thrive.

My parcel has a woods on the edges, and it wasn't too bad to establish paths in most of it, especially with a little help from goats to clean up the ground clutter. Similarly, there are trails in a nearby well established reserve, but you could easily go off trail if you had a reason too... Some bits are too dense to pass through, but most of it is fine. Your local forests likely grow a bit differently than mine though.