Think of a western move as an alternative universe, not history.
But "September Dawn" was based on an historical event. I also hate Civil War movies that show Confederate soldiers in the latter days of that war wearning immaculate and clean uniforms. By that time almost all Confederate uniforms worn by non-officers were shabby and worn down.
Likewise darn near any movie involving a computer: user interfaces all too often are slick hyper-stylized dreamworks having little (if any) resemblance to any GUI past, present or future.
And ... well ... movies in general are depictions of an alternate universe. People don’t talk that way (perfectly delivered witticisms, nary an “um”, “er”, or stutter save for deliberate effect), act that way (always timely and perfect), dress that way (good luck finding a matching outfit), work that way (heck, they never work), or even die that way (”BANG!” “thud!”, no mess). Whatever is shown isn’t history (past present or future).
Movies are in a sense a “seventh Platonic solid” - idealized perfection we think/talk about, but don’t exist.