So why didnt the monster die?

if its from deep in the ocean like the film (and its ARG marketing) implies, then that would mean it grew to a GIANT size in surroundings of insane pressure (millions of pounds of water pushing against it, a force which would instantly kill a creature of our size)

the point is something of that size that could survive at the bottom of the ocean would have extremely strange biological components

its skin would be vastly resistant to damage (i.e. why its hard to damage in the film).

there is no description of at what depth this creature was discovered, but if it does originate from an exceptionally deep part of the ocean then it could be comprised of organic material we're not that familiar with. there are some very strange creatures at the deepest parts of the ocean where no light reaches. and there aren't many of them so we don't really have a massive understanding of what is possible of a creature that far submerged in pressure

If i go crazy will you still call me Superman?

Because the movie would have been over almost before it started.

I just consider it a major plot flaw. It's shown as an organic creature of some complexity and thus it couldn't really be so immortal. But I like the film and let that slide. Otherwise it would have been killed pretty quickly and thus, no movie. This issue was probably discussed during the writing phase and they probably just said, "Well, it just can't be killed and there's no explanation".

Considering that for its size it would need to defy law of squares so whatever it was made of it would have to be material we have never seen before, or else it would collapse under its own weight. Considering that, it may be inpenetrable for all we know.

Because the people who made the movie don't know how weapons or physics work.