I thought it was b/c of the underground infrastructure necessary to anchor the retractable roof (I'm sure I've seen/read that before) but after a quick search this is all I found.
https://stadiumdb.com/news/2017/10/texas_1billion_superstadium_with_sun_blinding_players
So when we were laying out the stadium one thing we wanted to do was make sure the end zones were completely open. It was one of the keys to making sure the stadium did something that no other stadium has ever done before or done since. It transforms from an indoor, 80,000-seat arena to what feels like an outdoor stadium.
"That's the reason the end zones are open and the orientation is the way it is because that's the absolute best alignment to play out the final master plan and in creating the most value in the land around the stadium itself.''
The cherry on top is that Jerry never allows the stadjum roof open during Cowboy games b/c the roof traps in noise and he wants the "home field" advantage. Of course, when half the crowd is pulling for the opposition...