Model providers want to smoke test their models without having flaws end up on the news (think gpt4 having to get rolled back for sycophancy). Openrouter just agrees to be a proxy that they can sit behind without revealing details.

Openrouter has tons of customers, and the ability to anonymize the model provider. Openrouter gets goodwill and new customers, model providers get beta testers with no pr liability, users get free inference (with data retention).