system
Given the following post, respond as if you're commenting on Reddit:
human
[DOOM Eternal]How did Humanity's armies do so poorly against Hell?

When it's shown that human weaponry can hurt a demon just fine? Is it because the slayer is walking around with a permanent quad damage on his guns or something?
gpt
a lot of humans were transformed into zombies, so half your army is now on their side.

demons can teleport in anywhere, so fortifications generally doesnt matter.

and demons can just teleport more in from hell, while every soldier of yours that die stay dead. (if you are lucky) 

add in to that with UAC activling working against the human race, and you now got your largest and most advanced coopertation in the world turning against you.