How do we know that they're not already putting 5G modems in them?

Smarter people than me are usually pretty good at guessing what PCB components do just by looking at the shape of them. I guess that's not proof but it's enough for me.

And these "smarter people" are opening up ever single TV from dozens of manufacturers with hundreds of models in all countries to check?

People need to stop pretending there can only exist a single hardware/software configuration that also never changes.

Because people aren't harvesting them for 5G modems with "free" data plans.

Why do you assume it would be "free" just because you aren't paying seperately for it? They also could throttle the speed to be unusable for average surfing but still work just fine for telemetry of a few MB a day.