Mainly because theoretically we know hot to kill zombies
Unless zombies are/will be/could be a demonic force of death, we pretty much know that a head shot will do the trick or enough bullets to the body (if you believe video games like call of duty) or logically an explosion that completely destroys the entire zombie. We have no clue who or what aliens will do or be for all we know they could be flying nuclear and bullet resistant shelled sharks with death lasers and tracking robots and all that stupid crap. There are too many possible variations
If aliens arrive in warp drive or lightspeed spaceships there's a good chance we are as sitting ducks as you pouring hot water over an ants nest. You can't just take your rifle out a shoot a spaceship or alien designed tank. They've probably got super lasers and scanners to fish out every human living underground like a woodlice in the woodwork.