Okay, I was flippant before but I've got to address this seriously for a moment because it's making my writer eye twitch.
There's this weird internet behavior where people act like because they don't know about something, that something does not exist. "I never heard of you" isn't a dunk, it's an admission of ignorance. And that's not even a bad thing, because none of us know what we don't know. I like music, but there are artists who sell out stadiums who I've never heard of. It's a big world.
What I'm saying is there is a TON of American inspired fantasy out there. And I'm only talking fantasy set in America, about American things, not fantasy set elsewhere written by Americans because there's thousands of those.
Just me personally, as one dude, I've got three series that would be considered "Americana" fantasy. Monster Hunter International, which is a bunch of capitalist gun nuts, mostly from the south, killing monsters for fun and profit. The primary magic system is GUN. It doesn't get more American than that. (this series is up to like 15 things now, counting books, spin offs, and anthologies)
Then I've got the Grimnoir Chronicles (first book is Hard Magic), which is 1930s alternate history where magic appeared in the 1800s. I've got a dust bowl Okie who teleports and a hard boiled detective who controls gravity. It's extremely American.
And coming soon, American Paladin. Like, it's in the friggin' title. A modern vigilante roams the west, taking care of bad guys, human and other. It's cryptids, Missing 411, and Cowboys vs. Aztecs.
And I'm just one guy. I mentioned @DavidJohnButler because he's got a series that's not just American fantasy, but American EPIC fantasy. 1700s, big sweeping events, brilliant stuff. The first book is Witchy Eye. Check it out. But he's got another that starts with the Cunning Man, and that's a dustbowl Mormon wizard in Utah.
Hell, we've got entire sub genres of fantasy that are absolutely distinctly American. Urban Fantasy (which MHI and AmPal get lumped into, even though they're more rural) is primarily an American thing. The big dog of this genre is @longshotauthor with the Dresden Files being set mostly in Chicago. These books are a huge hit.
But Jim Butcher wasn't the first urban fantasy author, just one of the major hits that caused the genre to blow up. Before him there were authors like Mercedes Lackey and Tim Powers (that man is a genius) writing fantasy set in America, which would get lumped into that genre now.
Then there's the entire sub genre of Paranormal Romance, which is MONEY. The romance writers stomp the rest of us in sales and live in houses made of gold bars! However, that entire genre is still fantasy, and a giant chunk of it is set in the US and features Americans doing American things, written by Americans. @LKHamilton is a big deal mega seller in that genre, and her stuff is very American. She even wrote a story for one of my anthologies once that was about vampires in the Florida Keys, and the location was basically a character by itself.
Another fantasy genre which has an American origin, is everything Lovecraftian (which I do dabble in!). This gets lumped in with horror, but it's Cosmic Horror, which is really just fantasy with extra doom.
Sword & Sorcery is extremely American in tone, theme, and vibe. But those are usually set in fantasy lands.
Then you've got all the stuff that gets lumped in with Horror, which is often just scary fantasy. And there are some brilliant works that would fall in there by greats like Joe Lansdale or Dan Simmons. Hell, as much as I personally don't care for Stephen King, he's written a bunch of stuff that would be considered American Fantasy.
I don't know if people are getting hung up on certain fantasy tropes, and that's blinding them to everything else that's out there, but of course American fantasy is going to have different trappings than other culture's fantasy. We didn't have knights in shining armor here, but I created an organization of knights in trench coats and fedoras with Tommy guns which worked out great. (and I once stuck a dragon in Las Vegas, and he fit in surprisingly well).
There's probably a couple dozen writers on my TL who will chime in with their books that fit too, and I encourage them to drop those links.