It is possible but it needs people to develop the OS for each brand of TV and jailbreak the TVs to run another OS.

Most TVs are like phones in that they have a locked down system and their bootloader/BIOS/UEFI is also locked down which is what ultimately needs to be unlocked to allow another OS to be installed.

Why there isn’t?

Just not really many developers interested in creating an OS for it. Probably because of so many different brands and models of TVs that would require lots of work since each one is pretty different from another.