Stab in the dark here: HTML email is an attack vector that is under constant innovation by those who wish to exploit it. This likely leads to a not insignificant amount of investment by Google just to keep a step ahead of attackers.

This is true of many types of software but most software doesn’t provide a user friendly route to being easily exploited by malicious third parties. In this case that is rendering HTML from unknown sources.

That said, I still agree with your point. Google are a leader in the email space and any serious email service should be providing accessible ways of reading emails in all supported formats if they want to continue to be taken seriously.