It started out of necessity back when GMA was in second place but the show then, as it still does, believes a taped newsmaker interview is better because they can edit it down to keep the best parts. As someone else has mentioned, GMA puts a high value on story count.
From NY Mag in May 2005: Ben Sherwood says. “We don’t have the luxury of doing live interviews as often as they do.” As a result, GMA often pre-tapes interviews, then edits the segments; on a recent morning, Gibson taped an interview with the parents of children who had been roughed up by a school-bus driver—Couric went live with them later. “They’re getting a more lively pace by cutting out the deadwood in an interview,” says Andrew Tyndall. “It’s more about getting sound bites than the interaction between the interviewer and interviewee.”
The taped newsmaker segments became part of their culture and 16 years later they would still prefer to tape an interview at 6:50 so it can be chopped down to air at 7:15. Viewers don't care whether it is LIVE or not, as long as it is an engaging interview. (They also aren't watching all three shows simultaneously to see that one person is commonly on two shows at the same time)