Why didn’t he buy the mineral rights then?
In the west, it is often the case that the federal government came in and stripped off the mineral rights from the land quite some time back.
On our farm in Nevada, the desert land entry started with all the rights, and then just before the Dep’t of Interior deeded the land over to the first owners, the feds stripped the oil and gas rights off the land and held them ‘in trust’ for the ‘American people.’
After that, you cannot buy the mineral rights - you can only lease them.
Minerals rights aren’t always for sale.