Opportunity cost. If you can devote a few hours a day to each of, say, three gullible marks, you have a much greater chance of a payout than devoting a minute to each of 500 random marks.
That's referring to the time that the scammer needs if the recipient falls for their bait and initiates contact.
You only want the truly gullible to send that first email, or it would be a waste of time for the scammer to talk to all the people who wouldn't wire transfer their money a few days/weeks later.
Right, all contact after the first email has to be tailored to their responses. Even gullible marks usually need hours or days (at minimum) of building rapport before they're actually comfortable enough to be conned into executing a transaction. If conning people into directly handing you cash were automatable like phishing, you'd see a lot more con artists and a lot less of other crimes.
The initial contact is 'en masse', but the followups are all by hand. Time spent by the scammer to respond to potential marks is, in fact, a scarce resource.
Sending out the initial spams is very automated (and so cheap), but if you respond, they probably have a human handling than (maybe with templates, but still under human control), which isn't nearly as cheap, so they want to avoid wasting time on insufficiently-gullible responders.
This is precisely the point of the linked paper. Maximizing people who are initially attracted to the scam is NOT the best strategy for scammers, because most will likely be rejected at a later point, when it's costlier to the scammers.
The best strategy for scammers is to reject everyone but the most gullible targets as early as possible. Obvious typos would be suitable for this.
It's like how they say certain intro classes for majors are difficult to weed out students who would eventually drop out if they reached harder classes. The logic there is students aren't wasting their time taking classes they won't use when they switch majors, and the classes won't be filled up before students who will make it all the way can enroll.
This way the scammer isn't doing a back and forth for 2-3 emails with those who would eventually realize its a scam. They immediately weed them out so they are spending time on those who will payoff.