You can see precisely the parks boundaries as that is where the trees stop, rather astounding.
Our second day in Bwindi we found the Gorilla troupe we had been assigned to happily munching bananas from plants in the villagers' fields just beyond the park boundary!! Beyond, it may be "Inpenetrable Forest" but it is difficult to tell animals that they should stay there!
The Virunga Park in Rwanda has a similar "hard boundary" part way up the volcano with open fields all the way up to a 1.5 meter high stone boundary wall. We can't have walked more than a mile beyond it before we saw the Gorillas. In this case however the fields nearest the boundary seemed mainly to be planted with Pyrethrum which is not famously "attractive" to Gorillas! It has become major crop now for subsistence farmers there. Rwanda has the highest population density of any African continental country. It also has has an average of 6 children per couple. The country's population has gone up 5 fold since 1950, doubled between 1980 and 2010, despite the killing of 800k in the genocide and is still rising fast. . When one passes through a village one is surrounded by 100s of children. Somehow the country is going to have to generate "non agricultural jobs" for them all as there just isn't enough land for them all - and the gorillas, chimps and monkeys.
When we visited Kanha in India we were watching a Tiger just half a mile from the walls of peasant farms in which cows were kept - we were told that farmers got compnensated for any cows killed by tiger. In Kaziranga we saw a Rhino which had ventured beyond the park boundary and was chasing backwards and forwards after little kids who were teasing it and then climbing up electricity pylons.
So - wild life parks in heavily populated areas are a big problem - and wildlife parks in remote areas are also a big problem since such areas are more open to poaching. For many of these parks it is a matter of going to see them while you can!