I have a totally opposite rating compared to you. I think the idea for this movie is very interesting, but I dislike the lame idea of Hollywood movies which is, you killed the boss and all the all the enemies will die. Seriously? No better concept?

and to answer your question:
who cares where the alien came from. will it be any difference if they know where they came from? do you think you will try to figure out where the alien came from when they starting killing everything on Earth?

when you keep dying and dying and you're repeating it for few hundred times, won't you want to get a break? since you know that you can always restart by killing yourself...

This might not be the best from Tom Cruise but in my opinion, it's one of the best...