If a theater has a warehouse, they don't like to waste space storing sofas. If they don't... where would they? Chairs, tables, even beds can stack, hang, come apart for storage... but not a couch. So there are few stock sofas. Borrowing a davenport from civilians is also iffy - people miss their sofa if they loan it out. Besides, what if you ruin it onstage or in transit, and have to replace it? A sofa is bulky, awkward, and, inevitably, has to be carried downstairs. And turns out to be a sofa bed - built of concrete and cast iron.
But sofas are specific in style. They matter! A designer can't use any old thing. Yet loaners are rare, rentals are expensive, and reupholstering one to fit your scenic design is a major and expensive chore requiring Advanced Skills.
Except for this one, the "Climbing Settee" by Lila Lang. See it here at Decorfair. So cool I have to love it.