And not 100% of the land they own can be developed. Some of it is too steep, riparian zones, setbacks, cost prohibitive at the moment to gain access to some of these spots, etc.
When was the last attraction added that needed significant additional infrastructure added to get guests to the area? The station for Orion and orientation was placed where it was as the pathways and infrastructure was already in place and was extended/added on to. Same with Banshee, Mystic Timbers, etc.
Almost any new ride placed in the undeveloped areas would in all likelihood need new access created that would traverse under an existing ride, such as the access paths created for Flight of Fear and Action Theater. I am sure we could probably find a place to squeeze one in without that needed, but more than likely that would need to occur.
Doesn't mean they will not expand into some of the wooded areas at some point, but building where infrastructure already exists is going to be more economical. As rides/attractions reach their end of service life or the ROI is of diminishing returns, additions will occur in those areas first. I think an improvement outside of the "developed area" of the park would coincide with a new themed area with multiple attractions added to it. Or they go with the X-Base concept LOL and it takes how many years to get it to where it is today with a restroom and food place!