because, why send your own troops do do a job when you can send someone else to do it and call it a test to prove themselves?
And now I hate them. Well, time for me to blow up their base! I saved your lives and you repay me with MORE grunt work? TIME TO DIE!
Really? That's what you think it is? Grunt work? If you didn't wanted to do "grunt work" then why didn't you replace the Elder and kill the Van Graffs instead? Or is that still "grunt work" to you too?
Yes, damn the game for making you play more of the game! How dare they try to challenge you with content! Seriously though, they don't let any loser join up just because they found the bunker door. Also, other than not blowing up the bunker you at no point "save their lives" by any stretch.
By doing their missions, you find parts to fix their air filter. If you didn't do that, they'd eventually die unless they lifted the lockdown. >_>