Sales isn't about doing the paperwork of getting you signed up for a service. Sales is about convincing you to buy the service. Twitter doesn't have an automated system to convince companies to spend millions buying ads, they have salespeople, like every other sales company.

Yes most signups are done online via a self-serve system; however, there is a lot of money to be made if you can produce custom deals, content, and ad placement with bigger partners.

Guess how many salespeople Google has.

They are not selling accounts. They are selling advertising opportunities to businesses.

Hooo boy, we wish we could. Sales, much to my own inner-hacker's chagrin, is something done best by building relationships in person. You can put some of it off to a nice self-serve web experience a la Adsense, but if you ever want to make the big bucks (enterprise fuck-you bucks) you have to have sales people talking to real people who can write big checks and aren't Googling for self-serve solutions to their equally big problems.

Computers can't write sales copy or do a great in-person pitch yet.

You can, but the margins are much, much lower.