They can be. There's a lot of effort that goes into packing and sending a playfield. If it is NOT populated, then it is much easier. If it is populated with all the mechs etc, when it is a real pain, and sellers generally don't want to go to all the effort (understandable).

I once sent a fully populated playfield to NSW. I used a box from "Pack and send". They use a foam and a plastic sheath that is shaped to the playfield, so it becomes a sturdy and custom shipping box for that playfield. Convenient, but VERY expensive.