I don’t know either, I think it’s to do with how analogue signals work – the ‘static’ between channels is what scrambled analogue waves look like, the TV only uncodes the waves where there are channels.
Of course now we have digital TVs it doesn’t do this any more 🙂 (plus there are so many channels there’s barely any space between them!!!!
That is the aerial of the TV picking up electrical static energy, or electrical noise, that is all around us. If can come from other electrical devices, hot things, mobile phones, and even use. Depending on on where the TV is tuned to you are picking up a different bit of the electromagnetic wave spectrum, with only certain wavelength containing useful information, such as Eastenders.
I think I’ve heard that, in some way, when you listen to the radio or TV producing static, part of what you’re listening to is the leftover energy from the Big Bang, still resonating through the Universe in the form of microwaves. That’s a pretty crazy idea! Maybe more worth listening to than what you get when the TV is actually tuned in …