Because just like virtually everything in the embedded SoC ecosystem outside of the RPi competitor SBC crowd, the TV embedded board likely has a chip with little or no doc (with or without NDA), and unlike a Pi-ish SCB there's probably not even an unsupported, outdated kernel linked with a ton of opaque proprietary blobs hidden on an obscure Chinese language web site to try out.

Maybe there are smart TVs out there with a SoC that's been reverse engineered enough to do something with. If there is, that should be shouted from the rafters. But I kinda doubt it.