Free market at work.
I don't know when the last time western National Parks added to their ownership by buying large blocks of private land outside of inholdings. I would bet it was before I was born while Ike was President.
In fact, I bet Ike was a Major when it last happened.
Knocker is right.
Many people have this fear of any land that is close to a park will eventually become a park. In reality, adding lands to a park is pretty difficult.
Each NPS unit has a boundary (duh) and those lands within that boundary may be entirely govt or a mix. Those private lands within a boundary is known as an "inholding." The boundary is essentially the acquisition boundary. The agency has the approval from Congress to acquire any private lands within that boundary, all the agency has to do is fund it. The NPS cannot acquire lands outside of the boundary.
So lets say the NPS wants to acquire lands outside the boundary, what needs to happen is Congress will have to approve a bill that expands/changes the boundary. This sounds easy, but there have been largely bipartisan bills of similar nature in Congress for years. Once that happens then the NPS can acquire the lands.
So realistically if I had 1,000 acres of land next to Olympic NP outside of their boundary and I wanted to donate the lands upon my death, I couldn't do so. Congress would have to approve a bill to expand the boundary to include those lands, then once that happens the donation proceedings can occur.
There is an exemption which allows parks to acquire very small pieces of land without Congressional approval which will result in the boundary being changed, but again those are very small pieces of land.
National Forests and National Wildlife Refuges basically have to follow the same rules. They can't acquire lands outside of their boundary without congressional approval. The only agency without such boundaries is BLM. If someone donated 10 acres of land to BLM in downtown Seattle, BLM could take it.