(PRE-POST NOTE: After reading this post prior to hitting the button, I wanted to add this to say that I don't mean this snarky or as disrespectful as it could sound, it's just that the word you used (imagine) was a very good descriptor for me to use as well)
Imagine that it's not the 1800's any more. Imagine that a company may want to put a solar farm on some of that land, or any other number of uses that land could have if it were privately owned. Also, imagine the revenue to the government in sales profits and long term property taxes, income taxes from business conducted on that land, and other revenue streams to the government by privatization of the ownership of much of that land.
Now, I'm not talking about the Grand Canyon, or Monument Valley, or Yosemite National Park, or so on. There are, however, millions of acres of land that would not be risking damaging a national natural treasure by privatizing the ownership.