Ghosts can't die from poisoning because they're already dead, they can't be hit by normal things or fighting techniques because ghosts are portrayed as intangible, and don't have to worry about bugs because they're dead and arguably scarier than them. They break the concentration needed for psychic powers and bring fear, they're intangible and can't touch normal things in many depictions. Dark beats and resists them because ghosts are often seen as evil or malicious, where as Dark-Types are the evil type.
It's not very stupid, it's extremely easy to rationalise. As for Bug, it's meant to be weaker because bugs are seen as weak, but even then they're super effective on Psychic and Dark, both formidable offensive types, and resist Fighting and Ground, which are immensely present offensive types. It's just that a lot of Bug-Types suck.
And talking about intangibility, I really want a Ghost/Normal poltergeist with reverse Scrappy as an ability.