I am not from Oklahoma, but in the US there are common federal laws that apply across all states and set some high level regulations (there is a federal law that says brewing beer at home is legal with some restrictions), but then every one of the 50 states will have their own, often very confusing, sets of regulations related to alcohol (where you can purchase it? what days/times you can purchase? what licenses are required to serve alcohol? where can you share homebrewed beer? etc.).
In some states, homebrewing has only been technically legal for the past decade or so (though I suspect enforcement was virtual non-existent and you could likely purchase homebrewing equipment and ingredients in those states). Most states make a lot of tax revenue of alcohol sales, so most laws are in place to try and protect that revenue stream. Also, many states have lots of outdated laws on the books, or laws written to shield us from that demonic liquid that is alcohol. There are some "dry counties" in the US where you cannot purchase or serve alcohol, but I am not sure how homebrewing works there.
From what I can tell, Oklahoma requires a permit to brew beer at home that is greater than 3.2% ABW (~4% ABV). Why? Hopefully there is a valid reason, but I suspect it is just to set up a hurdle to make it harder for people legally brew beer at home.
https://www.homebrewersassociation.org/homebrewing-rights/statutes/oklahoma/