Because it makes them seem powerful. In Western culture, we read from left to right, and we watch theater and television that way, too. Our eyes end up on the right side of the screen—where the host sits (also known as stage left). In the theory of stagecraft, it's understood that a rightward placement telegraphs royalty. So no matter how famous the guest may be, sitting to the left makes him or her seem subservient. Late-night hosts also sit slightly upstage (farther back and slightly elevated) from their guests, which likewise reinforces the notion of a power imbalance.
Stage designers hold that guests make a stronger impression if they enter from stage left, crossing in front of the host and shifting the audience's focus ever so briefly. Perhaps that's why David Letterman—famous for the occasional cutting takedown—makes his guests march in from the weaker stage right. Colbert [when he was host of Comedy Central's The Colbert Report] plays with this dynamic most self-consciously. Guests wait in the interview area while Colbert makes his entrance. He keeps the focus on himself at all times.
No entanto, esses dias há exceções :
British comedian James Corden made his debut as host of CBS' Late Late Show on Monday, promising at least one big change to the familiar U.S. late-night talk show format before he even started.
"We’re gonna bring all our guests out at the same time, so all of our guests will sit together for all of the chat segments of the show," Corden told KPCC last week.
[...]
During interviews, Corden sits in an office chair to the left of his guests [...], who all share a couch. This literally puts him on the same level as the celebrities, theoretically allowing for more casual, more intimate conversation.
[...]
Interviewing without a desk, as well as inviting multiple celebrities on stage at once, is an intentional homage to iconic U.K. chat show host Graham Norton. On The Graham Norton Show, there is no desk. Norton sits to the left of his guests, who share a single long couch.
[...]
Bravo's raucous Watch What Happens: Live has perhaps the most unique set of any American late-night show. Not only does host Andy Cohen lack a desk, but he sits to the left of his guests. (And this is to say nothing of the on-stage bar.)