The basics first, because "BLM manages federal land" isn't self-explanatory if you've never had a reason to learn it. The federal government owns roughly 640 million acres of land in the United States, mostly concentrated in the West, and splits management of it across several different agencies -- the National Park Service, the Forest Service, Fish and Wildlife Service, and the Bureau of Land Management among them. Each agency has a different legal mission and manages the land under it differently. Most Americans have heard of national parks. Far fewer have heard of the agency that actually controls more federal land than any of the others.
The two agencies don't just differ in size -- they operate under legally opposite missions. The National Park Service exists to "preserve unimpaired" the land and everything on it, for people to visit but not use up -- that's the actual language of its founding mission. BLM operates under a completely different standard, set by the Federal Land Policy and Management Act of 1976 (FLPMA): "multiple use and sustained yield." Grazing leases, oil and gas leasing, timber, mineral extraction, wildland fire management, wild horse and burro herds, recreation, wilderness areas, and historical and archaeological sites can all sit on the same BLM land at the same time, by design -- not a failure to decide what the land is for, a deliberate choice to let it be several things at once. BLM itself traces to 1946, when President Truman merged the General Land Office (which dates to 1812) with the U.S. Grazing Service to create one agency out of two.
Why the size gap doesn't show up in how well-known each agency is. National parks are branded, marketed, and visited as destinations -- a family plans a trip "to Yellowstone," not "to a National Park Service unit." BLM land rarely gets named that way in public conversation, even when it's the specific agency someone is standing on. Red Rock Canyon, 17 miles west of the Las Vegas Strip -- nearly 200,000 acres, three million visitors a year, a scenic 13-mile loop drive through sandstone formations up to 3,000 feet high -- is BLM land, not a national park, even though it functions exactly like one to most people who visit it. The same land supports wildly different real uses at once: mineral leasing and grazing on one BLM parcel, world-class rock climbing and hiking on another, an off-road drive through the mountains, or a two-mile hike in with a kid and a dog to reach a fishing spot, then two miles back out to the cabin at the end of the day -- all legitimate, all real endurance in their own different ways, all happening on the same kind of land, under the same multiple-use mandate, without most of the people using it ever registering which federal agency actually manages where they are.