Most people are stupid enough to not change their posting styles or behavior when they use their second, third, etc account. Things like font size, type, color easily gives away you are. For example, a member gets banned for harrassing all the mods via PM asking to chat all the time. He comes back under another name and does the same, so it's quite obvious.
The second reason, is that there are lots of snitches in this place. You'll be surprised how many of your "friends" are willing to turn you in to get on the good side of the staff. So the PM to your friends saying, "Psss, hey it's me I'm back don't tell the mods" is the kind of stuff that gets you in trouble.
Third reason is that they have certain technologies they have that they can use. But that is a line I won't cross and expose.