I think the reason why people think written communication is ineffective, is because most people just can't write.
Personally, I think I talk like I write, which makes my oral communication less effective. As I'm talking, if I think of something else to say, I have to revise my previous statements so that the listener ends up with the same mental picture as me. This takes time and words. When writing, though, I can just delete the thing that I didn't really want to say, and replace it with the better version. This saves the reader time and makes me look better. I think.
(Maybe this also makes me odd, but I feel the "good feeling of social interaction" both after speaking with someone in person or after communicating with them via writing. Writing emails to my favorite girl du jour makes me as nervous as meeting her in real life.)