We can't have accurate costumes, they need to look like Lord of the Rings / Game of Thrones or people won't like what they're seeing and turn it off!
We can't bring in any ambiguities because this stuff happened thousands of years ago and we don't have and never will have the full story, people get frustrated and bored if they have to think and aren't spoon-fed a simplistic dramatic narrative! They don't like it and then they watch something else! So just make shit up. As long as it's dramatic and reminds people of LotR/GoT, we're good.
Battle scenes gotta look like the Avengers as much as possible (considering the subject matter) because that's what people like and if we filmed a more accurate battle scene, people probably wouldn't like it and would stop watching!
It's a deep insecurity about their ability to tell an entertaining tale without following the basic conventions set by Hollywood and places like HBO, and a condescending view of their audience that they don't want accuracy, they want another dramatic action extravaganza of the kind they've already seen dozens or hundreds of times in the last 20 years.