That's not to say that Visa hasn't worked toward favorable regulations, but the biggest problem with alternative payment networks (to Visa/MC/Discover/AMEX) is that most retailers don't take them which means most customers don't want to set them up. If most customers don't have those payment methods, there's little incentive for a merchant to setup those payment methods.
With OpenAI, there isn't the same network effect. I don't care what LLM you're using or what LLM Walmart or anyone else is using. If Anthropic or Google start offering better/cheaper LLMs, companies can simply switch over. Maybe some integrations will need to be rewritten, but that's a lot easier than getting hundreds of millions of customers to sign up for something new. Plus, companies like Google are simply implementing the OpenAI API so that integrations don't have to be rewritten.
By contrast, if another company launches a payment network, it isn't enough just to convince a company to switch. You need to convince millions of consumers to switch - which would require convincing enough companies to switch.