The plain answer
For the attendee, most webinars are free. You register with a name and an email, you receive a way in, you arrive, you listen, you leave and nobody sends you a bill. This is the ordinary arrangement and the reason the format gathers rooms: the barrier to attending is a decision rather than a payment, and the decision is cheap.
For the host, nothing about it is free. The session is the visible part of a long invisible preparation, and behind that sits a stack of tools that each cost something monthly. When someone says a webinar cost nothing, what they mean is that the cost was moved from the door to the host, and the host absorbed it as marketing, teaching or goodwill. That is a reasonable trade, but it is a trade, and pretending otherwise is how a session ends up under-planned.