Those companies all spend money to create and/or license content. Twitter seems to want users to pay $8/mo and continue to see ads for the privilege of creating the content that brings users to Twitter?
Companies aren't voluntarily charging barely enough to cover costs - they're being forced to do it by competition. Normally, they'll charge you as much as they can get away with.
It would be news if Twitter, or anyone else for that matter, decided to voluntarily charge less for the sake of fairness to the users.