.. because they believe that they'll get more money from the efforts of these people than they cost? If they can sell ads that people wouldn't buy without a person on the other side, they can be worth the cost.

Twitter has people to sell adds to companies that use people to decide to buy ads. If automation makes it easier to use fewer people to buy ads, then twitter and other ad companies would likely sell ads with fewer people in the mix.

As an aside it's odd to think of twitter and google etc as a kind of modern successor to the MadMen ad house. I guess if you were being pedantic, they're more like the modern version of what the network TV commercial industry was in the 60's.

Selling advertising online could be automated for the most part, unless Twitter really does offer personalised offering to ads buyers—which I'm not sure is the case.

At a certain spend level, advertisers want to have a single human that they can call when they need something/something goes wrong ("a single throat to strangle").

Providing this dedicated connection is also an opportunity to upsell them on more ad products.

I did this work at Google for a couple years at the start of my career. They are an exceptionally analytics driven company, and would not employ these kinds of people if there wasn't good ROI