How does WWE earn money with an empty audience?

BusinessInsider.com reported on this in an article back in 2015. The statistics were culled from major North American sports teams.

The amount of revenue generated by major North American sports team from the selling of media rights (TV, radio, internet, etc.) is projected to surpass the amount of revenue generated from ticket sales in 2018, according to a PricewaterhouseCoopers report (via Sports Business Daily). As a result, television ratings have now become the driving force behind sports, even bigger than actually bringing fans to the games.
  _{source: Why TV ratings in sports are more important than attendance - Nov 17, 2015}

And again three years later.

Media rights — or the fees paid to show sports on TV, the internet, and other distribution channels — will remain the largest and fastest-growing bucket of sports revenues in North America, according to PwC’s Sports Outlook.
  _{source: Media rights will continue to propel sports revenue - Nov 28, 2018}

Although the WWE should probably be considered a secondary market, they have long embraced home Pay-per-View and licenced satellite broadcast to bars and other public venues for 'championship' events. Cable sports networks broadcast matches of lesser importance on a regular basis and these would be subject to contractual fees as well.