In the USA parks are different than reserves or forests. National Parks has the stereotypical park ranger and guides, plus usually lodging and visitor centers and stuff. There are also national forests which probably have some trails and stuff but it's mostly "here's some land, good luck out there". There are also wildlife preserves and stuff.
National parks in Australia are essentially conservation areas that are protected. Maybe more like the national forests, parks and lakeshores combined.
its because a big chunk of the US that drives through the entire center area of the US, including northwest Texas, is the great plains. there's basically grass for several thousand miles