Why?
That’s why this thread has been discussing the Resolution of 1780. When the feds dispose of agreed on land in a US territory to a new state, people like me and others interpret that resolution as requiring the feds to give up control of everything in the new state.
But possibly as an explanation for the “federal” land in question in Nevada, what may actually be seeing is this. The feds have historically held onto land in several states when they were accepted into the Union, arguably as an unconstitutional way to bypass the Constitution’s Clause 17 of Section 8 of Article I, and also the 5th Amendment's eminent domain clause, as opposed to having to later buy property in that state for a constitutionally limited purpose.
Steven Pratt explains his research of this issue in the following video.
Steven Pratt, Bound by Oath to Support THIS Constitution