I created a personal fan theory that we never saw the actual aliens in the movie.

What we saw were biological constructs created by the aliens. It makes all of their weaknesses make more sense.

The creatures are vulnerable to water, on a planet full of water. It makes sense if this was an intentional part of the design. The creatures were created for short term missions. If they were captured or killed by the primitive locals, their bodies would degrade and leave nothing useful behind.

Same reason they have no advanced technology. Nothing for the locals to get their hands on and reverse engineer.