Indeed, in the BLM's own words, there is ten times more land available than needed per animal. Still, as of August, the BLM predicts "excess" horses will eat grass and cause weeds to grow on federal land. The BLM contracts two agencies to get "scientific" data on the horses. Since both agencies list their priority as using federal land for profitable government business, their research is biased against the horses.
The first agency, USGS Fort Collins Science Center (FORT) is paid by many government agencies and policymaking groups to generate evidence to back the client's desired policy. FORT does wild horse population and fertility control studies for the BLM. These studies are used to say it is right to get rid of wild animals in order to expand profitable government businesses, like solar and wind power.
The other agency, the USDA-Animal and Plant Health Inspection Service (APHIS) says it "provid[es] wildlife damage management assistance to protect agriculture, natural resources, property and health and safety," meaning the BLM pays APHIS to show them how to get rid of wild horses. There is no direct public control of APHIS, and no clear mention of their role on BLM web pages on horse roundups.