This is because the TV outputs audio via ARC, or its internal speakers + Optical. Neither of those methods supports TrueHD or any lossless audio. ARC only supports DD+ with Atmos metadata, and Optical won't support Atmos in any form. Your option would be to find a device that connects to your receiver by HDMI, like a 4K Blu-Ray player, which can play media files from an external drive. Or, y'know, buy the movies legitimately.