They gain a lot thanks to network effects. Non-paying customers contribute to viewing numbers and engagement (likes + comments + subscriptions). But most importantly, they bring others to the platform by sharing links to videos and discussing them with others.

See [1]:

> In 2013, the boss of Time-Warner (which owns HBO), Jeff Bewkes, declared that piracy was: “Better than an Emmy” because more people watching the show inevitably led to more people deciding to pay for subscriptions.

[1]: https://theconversation.com/game-of-thrones-for-hbo-piracy-i...