The answer to that questions goes a long way to addressing some of the more ridiculous implications of the approach you're taking.
For example ...
The U.S. Supreme Court has long held that the Federal government has the authority to own and retain land in the various states. The reasons for this are obvious when you consider the details of the Federal court cases that resulted in these decisions. Two of the biggest factors here are:
1. The Federal government signed treaties with native tribes in these western states that pre-dated the existence of the states. These treaties were signed when the Federal government was in a role of managing a territorial government, and in many of these cases the current state borders do not match the original territorial boundaries. This last point was the basis of a U.S. Supreme Court decision in 2019 -- where it held that a member of the Crow tribe who lived in Montana still retained hunting rights in Wyoming under the terms of the 1868 Treaty of Fort Laramie. The notion that the establishment of statehood renders any previous legal agreements null and void would seem to be preposterous on its face -- especially for anyone who recognizes the importance of clear land titles in maintaining the rule of law in any jurisdiction.
2. In western states, the original motivation of the Federal government in holding title to land (aside from military considerations) was to protect watersheds and minimize conflicts between states and territories over water rights for agricultural uses. Folks like me who were born and raised on the East Coast take these sorts of things for granted because water has never been a problem here. Rainfall is abundant and supports every agricultural use we've had here in the last several centuries, but for western states that's a whole different story. There are hardly any natural rivers that flow within a single state, and the availability of water for ranching and mining activities was a major bone of contention in so many land disputes in the 19th century -- involving both settlers and Indians alike.
- signed treaties with native tribes:
Tribal lands were never considered part of a state. These “reservations” are land “reserved” for Indian Nations supervised by the feds, not the states. Nothing unconstitutional there, because the land belongs to the Indians, not the state.
- water rights for agricultural uses
The feds have power based on the Commerce Clause to regulate commerce between the states where necessary. It is not necessary for the feds to own the land, just to regulate where necessary, so as you say, “to protect watersheds and minimize conflicts.”