Many movie producers don't know what is correct and don't care. The believe that the average viewer is not concerned and doesn't know either, so they just use what's available to them. It's annoying to those that like history and correct details, but 99% of the viewers could not tell you what years WW2 or the US Civil War was fought, can't tell you anything about the individual battles or why they are important and could not care less (total ignorance and apathy of their history is the largest reason the "left' can make inroads into our nation and destroy individual rights I might add)
look here:
https://www.youtube.com/watch?v=GoUSx7RF-28
https://www.youtube.com/watch?v=26-c4lZLPZc
And these are not the exceptions, they are the rule.
We have all seen Civil war movies where the combatants are using Colt SAAs and Winchester lever actions.
Patton had the Germans using M48 Patton tanks and the US using Walkers.
To some degree I can understand the problem. Now, in the days of CGI, the excuses for such lack of accuracy is dwindling, but when Patton was made, original running German Panzers and even M4 Sherman, Grant and Lee tanks were very very rare.
As far as small arms go, the excuse is not as valid. To some extent it may be forgiven, but any gun shown up close and with details that can be seen should at least be a good plastic molded replica of the real deal in my opinion, but I am like you guys, knowing what I am seeing for the most part. Most people don't.
The large majority of TV and movie watchers would not know the difference between an SMLE, a Gras or a Mauser. The large majority of them can't tell you the difference between a flintlock and cap-lock and don't know what periods of time either one was common.