Primarily because it seems logical that the government would own the largest amount of land in a country like the United States. No individual or institution would ever come close to the sheer amount of land the Federal Government obtained by treaty or by conquest.
Not who you're replying to, but the US Govt does own a lot of land that wouldn't be useful to anyone else. Some rural and desert plots sell for less than $1000/acre.
While that may be true in some cases, the US Govt also owns a huge amount of productive land that are today used under lease for grazing and other purposes, or could be used by others for purposes that the US Government forbids.
I am not sure what the exact break down of useful vs non-useful is, but I know most of the land in my state that is owned by the government could be used for other things.
Keep in mind to we are not just talking Bureau of Land Management, but also Park Service, All Federal Agencies many of which have abandoned, or under used facilities in many towns and cities through out the US, Military who owns large sections of land that are non-active bases, etc etc etc
Several states have even attempted various law suits because the federal government is preventing economic development of the land