Why would any buyer go through a real estate agent?

[3d]

Nowadays, with public MLS (lite) sites, Trulia, Craigslist, etc. I can pretty much find all the information i need about a house before i even step foot in it.

Address, how long it's been sitting on the market, lot size, school district, and even in some cases when it was bought and how much it cost the seller.

Is there anything from preventing me from just leaving a note on the sellers mailbox to let them know i'm interested. Call me for coffee and a time to see the house?

I know sometimes at OpenHouses the agent will have you sign a contract saying that if you are interested in the home you have to go through this particular agent and no other. And you can't bypass the agent and talk to the owner directly.

But what about a listing online, that clearly shows the address?

I guess security for the home-owner is a concern. But you encounter that with any OpenHouse.

If i was a seller i would love to bypass that agent and save $12,000 (3%) on a $400,000 home.

Am i missing something?
When a seller agrees to go through an agent are they forbidden to sell the property themselves anymore?