No one said that, but it doesn't have to be bad. And with the BLM, it is.
Farmers have an inherent interest in keeping their own land profitable, which is an incentive that never applies to leased BLM land. Same with ranchers, which is what BLM land is more commonly used for.
> I cannot comment on how poorly the BLM manages land
That should be your first step. It should be noted that ranchers do NOT want the land to be privatized, because it's less exploitable.
http://wildhorsepreservation.org/media/ranchers-want-our-pub...
Agricultural land is not going to be perfectly managed anywhere anytime soon, if that's what your standard is, but private property is managed less terribly. It should be obvious that people are better incentivized to care for their own property than the government's -- why is that so hard to believe?
You have only even attempted to demonstrate that some private farms are mismanaged -- where have you shown that the damage that objectively occurs on BLM land is less bad?
To say that agriculture is good for the land on average is just an outright lie.