You can’t pick up a signal that’s not being transmitted towards you. Global television coverage is not universal coverage. A typical TV transmitting antenna emits a flat plane of signal. This plane will intersect a distant planet for a brief time twice daily, due to the Earth’s rotation.
Satellite television is less useful for extraterrestrial detection, because its antennas are optimized to send 100% of the signals down to Earth, not into space. Go look up their antenna coverage patterns.
Furthermore, digital TV signals are pretty much indistinguishable from white noise on a spectrum analyzer.
All TV receivers are designed to receive exactly the type of signal that’s being sent to them. The range of possible frequencies and modulation patters is vast, which is why SETI spectrometers have millions of channels. They are operating under the hope that there’s some repetitive pattern to be detected somewhere.
But it takes a lot of resources to generate such signals, and commercial broadcasting is the only industry on Earth with the funding to do it (why would the military advertise its existence?) And as I’ve explained above, the TV industry is optimizing its transmissions so that no signal energy is ‘wasted’ by sending it to space.
Who knows what other civilizations would do in these circumstances.
