I didn't even know they existed outside of the tutorial, I stumbled upon one on LVL 2 and solo'd it - I can only imagine how stupidly chaotic and fun it'd be to do one with a full group on the higher difficulties.
I think it's because of what the mission is supposed to represent. The flag raising is filmed by a drone because it's propaganda. It's supposed to show the Helldivers conquering land deftly and confidently in their shiny badass armor, bristling with guns at the ready.
Filming a flag raising on a helldive mission as enormous swaths of enemies kill and throw the Helldivers around like basketballs is not something you wanna show as confident, powerful propaganda, so it doesn't logically make sense.
Yes yes YES! Me and my regular fire team have been saying this for weeks. We sometimes but on our fancy gold braided dress uniforms, equip the salute and go raise a flag, but it's so dead and boring. I want to have to fight crazy hoards and plant a flag in a bile titan or factory strider's corpse.