Because they're tired of paying $80/mo for cable because it's the only way to get a couple dozen channels they want, but the cheapest bundle that includes those channels comes with hundreds of other channels they're not interested in. Whereas the streaming services offer more granular selection which lets you pick and choose those channels you want for $30/mo, because they're not bundled with a bunch of expensive sports channels you never watch.
Random story which may elucidate why this is wrong:
I played World of Warcraft back in the classic days, and used to get pretty miffed when I saw +Spell Damage on a piece of armour instead of +Frost Damage; after all, why waste precious stat points on all those elements my frost mage wasn't going to use? Bollocks to that +33 Spell Damage item, it should ditch the other elements and become a +40 Frost Damage one.
My view was a bit naive, I'll admit. The game was balanced around the amount of bonus damage the existing armour provided; if it were altered to give me an advantage, which seemed possible (+single element damage seemed to use fewer item stat points than +all element damage), then they'd have to rebalance everything around that; probably by lowering the base damage of my character, which would suck.
So while you see this bundle which has Awesome Channel + 30 other channels you don't give a hoot about, the cable company is aware. It's bundling those for convenience on their behalf. If they removed the 29 channels you don't watch they wouldn't reduce the price, because it still costs them the same to deliver those channels that you do watch as it did previously. The others are sort of a bonus.
It's a bit weird, and easy to feel like you should be able to optimise and pick just the bit you want to get ahead, but it seems likely that the removal of channels from a pack wouldn't affect the price at all, because you're only paying for what you use - you're just lumped into a group with others who pay for the [different] parts that they use.