They use "predictive dialing" and have the settings set too aggressively/too few reps. The predictive-dialer robo-dials, and then when somebody answers, they hand the call to an available rep. But sometimes (or maybe most of the time) there is no available rep.
The predictive dialer is supposed to predict the call rate and availability. Some probably aren't "predictive" at all, just dialing at a fixed rate. And then somebody goes to bathroom and screws things up.
The scammier the operation, the more likely you are to run into this.
Their response rate is so low to begin with, that they don't care if they lose a few (or half) due to the delay.
If you were dialing for free money you'd do the same - turn the dial to 11!