When Hollywood transitioned from the '80s to the '90s, technology was a huge participant when the rules were changed. Suddenly, horror wasn't as important as "looking good" in science fiction, and plots tended to be put aside for the sake of a better "look and feel."
Regardless, they're far from being bad films. They're survivors of an era where special effects could be the main character and these didn't have the greatest budgets. As some of these show, sometimes it's not about the budget size, but how you actually use that size.
I think I love this list, suddenly want to re-watch pretty much everything in it except maybe The Rocketeer (just don't like superhero stuff much, or Disney). Actually did re-watch The Arrival not long ago, still one of the best "they're already among us" movies that isn't They Live (and also Charlie Sheen's best role) as far as I'm concerned.