It's user base and brand. Just like with Pepsi and Coca Cola. There's a reason OpenAI ran a Super Bowl ad.
Most "normies" I know only recognize ChatGPT with AI, so for sure, brand recognition is the only thing that matters.
Yeah but cheaper alternatives (and open source and local ones) it would be super easy for most of the customers to migrate to a different provider. I am not saying they don't provide any value, but it's like paid software vs open source alternative. Open source alternative ends up imposing, especially among tech people.