Why was the nail left in the stair tread ?

Rating: 3/10 (5 May 2018)

SPOILER: Cliched and predictable , why was the nail Emily Blunts character trod on in the stair tread in the first place, and after she trod on it why was it still there ? could have bent back and forwards until it broke , silently. Before she trod on it .
How did the monster literally break out of a grain silo , but couldn't rip the roof of a small truck ? Why not wear slippers or soft soled shoes ? Who in the right mind falls pregnant in a post apocalyptic world , where the monsters kill by noise, hint , babies scream... a lot. Finally the military might of the worlds collective might and intelligence couldn't work out high pitches frequency's killed the rather cliched looking aliens.. really ?
This was as bad as Titan with Sam Worthington , an insult to the voters intelligence in my humble opinion .