And ext has plenty of documentation and they haven't implemented that either.

So I think keeping down the software complexity is the more likely method. Down to

1. The limited software investment these companies make (often just buying and reskinning firmware from AMI)

2. The limited space available on the ROMs due to hardware cost savings (e.g. companies have had to drop GUIs or support for less popular APUs to add support for new generations of mainstream CPU in firmware updates)

Although very complex and very undocumented, NTFS has actually been adopted very widely and quite reliably. Most of the pre-smart TVs and set-top-boxes can read FAT and NTFS USB drives which makes NTFS the only choice if you want movies exceeding 4 GiBs.