I phoned a friend, so to speak, who is Manager of Line Maintenance for a US airline at one of its hubs... so the inflatable belts that have extenders are specially designed, much more costly for the base seatbelts, and then of course for the extenders. It adds additional cost and complexity as that's another spare part that has to be stocked, and must be on board the specific aircraft that require it. Long story short, do you want to be delayed in Wichita because someone on the inbound flight walked off with one, and now they don't have the required number on board, none in stock, and have to get contract maintenance out to do paperwork or inop some seats?