I don't understand why they don't just charge accounts with more than X followers. High profile brands are (currently) getting a massive amount of free advertis

I don't understand why they don't just charge accounts with more than X followers. High profile brands are (currently) getting a massive amount of free advertising. I'm sure they would be willing to pay to keep up their profile. See Mailchimp, free up to 2k subscribers, paid after that. It's so simple.