Cost to buy that content
does go up with the number of users, the typical licencing agreement of a cable company to distribute a channel has pricing of $x/subscriber.
There can be some other conditions, e.g. if you distribute it to less than 100k subscribers then you still pay for a full 100k; or you pay $x for every subscriber that you have, no matter if you show them our channel or not; or you can distribute the nice channel A only to customers who also have our (not so nice) channels B, C and D, etc... but the core principle is that you do pay for content per subscriber.
Furthermore, the big content companies produce a lot of content in all genres, and they're (obviously) focused on their average revenue per user - and if they can get $x ARPU for their bundles, where the demand is driven by a few key channels (e.g. they expect everyone who wants A or B to pay for the whole bundle) then any a la carte unbundled packaging that would allow A or B separately makes business sense only if the a la carte customers would on average pay at least as much as the whole bundle costs. There's no extra cost to the content owner to give the whole bundle, rather the other way around (your 'shit channels' can get more advertising revenue since some of those who got it in a bundle would watch it a bit), but there is a loss of revenue if you start paying less for the channels that you do want.
Advertising covers the costs for the many shitty channels, those can be got/distributed nearly for free, but things that people actually want on their cable are quite costly, especially sports.