It's not like Alien is all that original itself.
It basically straight rips off Planet of the Vampires for the first half of the movie, and It! The Terror from Beyond Space the second half.
Alien just perfected an already well used formula with better writing, characters, set design and creature effects. As well as how they brought the creature aboard.
If you ever watch Planet of the Vampires you might be super surprised to see how well Alien "mimics" its first half. When I finally watched it I thought it borderlined on plagiarism.
And I can't really see how Alien and RE are that similar. RE would have more in common with Aliens.