A landmark in sci-fi, however some big issues with plot lacking realism

Rating: 7/10 (18 February 2018)

First of all, for a sci-fi movie made in the end of 70s - and it doesn't look and feel silly for the most part - it deserves a high rating just for that.
We can only imagine how innovative this looked almost 40 years ago.
Great flick overall, I only have some issues with the plot which takes away from the experience - it's just too damn unrealistic. By the logic of this plot, the human race has to be the dumbest space-faring species ever, just too stupid to survive.
Heavy spoilers:
Stumble upon an unknown weird sounding signal in deep space, which noone understands. Witout analyzing it, assume it's a distress signal and go to investigate this with an unarmed cargo ship with a crew of 7. Perhaps just to observe a bit closer? Yeah OK makes sense so far.
Find out the signal is coming from an unknown planet in an unknown solar system, and immediately decide to land on that planet without further observation or even consulting Earth command or anything. OK, getting sketchy...
After landing and partly wrecking the ship immediately go outside in an alien world you know almost nothing about - without any idea who or what you'll be up against. Find an alien structure undoubtedly created by an intelligent lifeform, and immediately climb into it. So yeah then find a huge alien corpse who by the looks of it died a violent death, all the time touching everything.
Then go climbing into some dark caves - sure why not! Then find a field of huge eggs and go stuble around them and of course go touching these also - might as well... if you already did all the above!
In the meantime back on the ship the crew learns that the signal looks like a warning. And although they have live video feed from the team outside, they can't or won't warn the team about this crucial information.
And so on and so on... Ripley is the only one with a sense of reason and she does warn everyone along the way... however it's just not enough for all this to make any sense - her very reasonable warnings should already be in the protocol in such cases. Umm.. like shouldn't the safety of the crew and ship be prio 1 etc?
The lack of realism also extends to acting here and there - after 2 crew members horribly killed, the captain is still calm as a coma like it's an everyday thing his crew members get butchered by deadly aliens on his cargo ship. He's talking up his plan like he's ordering a damn pizza - without any signs of stress or worry whatsoever.
Sure it's sci-fi and about the unknown, but in order to have a better impact an be more submersive as a movie, the plot should still makse sense in the areas that ARE known - like human behaviour and probable space-faring protocols that should follow some common sense.
The same events and suspense could have been backed up by better screenwriting - like they would have followed protocols which made sense in the first place - but some things went wrong etc. And the immersion and suspense would have been better because of increased realism.
So 7/10 for lack of realism, for an otherwise great ride into the unknown.