As another commenter notes, because you get access to a lot of functionality for free. And other providers are also providing free alternatives. The ratio for their free/paid tier is about the same as YouTube's. And like YouTube, it's not that YouTube isn't providing great value, but rather that most people get what they need out of the free tier.

The better question is what if all LLM services stopped providing for free at all -- how many paid users would there then be?