A custom API is the only way for a platform to extend its native E2EE sessions and features to other platforms. Making those APIs completely open would become a major spam problem, which would likely end them up in the same situation as SMTP, where small servers are blocked-by-default by big providers.

Interoperability by agreement between legitimate messaging services, using custom APIs is the only realistic and secure way to accomplish this.

But WhatsApp is already completely open for spammers. They can use the secret API or screen-scrape WhatsApp itself.