Why doesn't the MSQ have quests where you participate in large open battles?

[Jeeqbit]

Prior to Stormblood, they could not have as many NPCs on the screen. You will notice Stormblood dungeons are flooded with NPCs compared to prior expansions. This was due to abandoning PS3 support. In Shadowbringers, they abandoned support for 32-bit and we saw a lot more special effects after that.

It does happen in a solo battle after beating Bardam's Mettle where there is a giant battle for the Azim Steppe. You also see lots of NPCs fighting in the background of the Ala Mhigo dungeon (I actually had to upgrade my computer at the time due to it), which I think is meant to be the battle from the Benchmark (but in a different location).

It also happens in the Ghimlyt Dark, where the allied forces are fighting Garleans in the background. Shadowbringers released Bozja, which had fighting everywhere and was trying to create a battlefield. Endwalker sent the allied forces to Garlemald and we participate in a giant solo battle there to win what's left of it, but there are NPCs fighting everywhere.

They put a lot of escaping villagers and animals in Holminster Switch and Amaurot. The Heroes' Gauntlet has some battles taking place as well, but more sporadic due to the nature of the conflict. And there are some giant solo battles against Eulmore in Shadowbringers with fighting going on all around.

In the Benchmark, they don't have to account for possible players actions or other players and it's really just a test of the limits they want your GPU to manage. It doesn't mean they will actually push to those limits unless the story wants it. And they use the Benchmark to throw you off a bit so as to not spoil the story so that's why the battles don't actually happen.

I do agree that they could have had more solo instances and battles in Dawntrail though.