I have long wondered the same thing. Why on earth would they think this would benefit them? Also, why do people think this is going to happen swiftly? It's just completely nuts.
I think it has a lot to do with cost in this case.
The problem with cable is the infrastructure. It's expensive to install and even more expensive to maintain. Dish isn't too far behind in that it is a little cheaper but there's still infrastructure that needs to be dealt with at the customer level.
5G is super cool because it's a lot cheaper for a company like Comcast to piggyback onto a digital signal. Now you're back to a broadcast model where you can fire a digital signal right at your customers. In cities, that's pretty easy. Out in the sticks, it's getting easier.
I live in the middle of nowhere. I cannot get cable run to my home but the cell service has improved dramatically in just the last half dozen years. There is even talk of providing wifi to the community via a nearby tower that is currently in the development stages.
This isn't a huge leap guys.