Originally Posted by skywardhunter

Mass and balance. It'd be aerodynamically inefficient and a safety hazard.
​​​​​​The alternative would be alternating every few rows but that'd make for jigs in the aisles and loss of seats at the switchover points.

It's also visually displeasing. The manufacturing inefficiency could probably be overcome or would be minimal. Also it'd have lots of pax feeling like they "lost" unless the get the set of 2