That's because Ridley set it up as an intended trilogy - and you don't give all the answers in part 1 of a trilogy.
But if a trilogy (or even a sequel) didn't happen, Ridley also wanted to frame things in such a way that the answers were right in front of you - you just had to bring your brain into the theater with you and "read between the lines."
Right, if you don't like Prometheus
and point out its many flaws it's just because you're too stupid and close-minded to comprehend its obvious brilliance. Or at the opposite extreme if you disliked the Transformers
films it's because you're unreasonable and expecting Shakespearean-quality deep, intellectual films. Those two complaints never get old.
I can come up with a hundred theories to try and make some sense out of the script and directing too, but at some point I have to ask myself when I get a writing credit because I'm doing a lot of work trying to salvage this train wreck of a story. I'm happy for you that you enjoyed the film for what it is and prefer to fill in those blanks yourself, but that doesn't make people stupid for thinking otherwise or not tolerating what they feel are plot holes and nonsense characters.
Movies have to be internally consistent even if they're part one of a trilogy. You can leave some mysteries hanging, but not everything, and especially not if you're going to couple that with poor pacing, badly written characters, and pseudo-intellectual mythological references that go nowhere.