Why do flats never seem to have decent soundproofing?

Is it even possible to soundproof effectively so you can't hear someone above you or below you?

I can hear the man in the flat beneath me banging doors and moving things around, it gets on my nerves.

In another flat I had I could hear when the man downstairs coughed. Yes, really, coughed! I noticed he did it sometimes when I was walking around the flat quite a bit, doing chores.