No, they're afraid that other web browsers will fail to implement features that Google needs unless Google has a significant say in the standardisation process by means of having a web browser. They'd be at the mercy of other web browsers for those features otherwise.
Do they not get a 'significant say in the standardization process' because they don't have a web browser? And if not why should those who have a web browser get a say? Much less a significant one? The only way this matters is if you extend the capabilities of your browser outside the standards process. So if everybody would follow the standard then that wouldn't be a problem.
No more Microsoft sites that only work with IE (remember Active-X/COM objects?) no more Google using features that are only present in Chrome to cripple Google docs on Firefox.
Google can pressure others to agree to follow a standard by being an implementer, and they do. (There is practically nothing accessible from a general web page on Chrome that isn't defined in a standards document or draft standards document.)
We, unfortunately, don't live in a perfect world where web developers can unilaterally force browser implementers to implement standards - Google is a large company that heavily depends on the web, and needs to be able to do that. For the most part, other companies can contribute to Firefox to get that, but Google likely decided the politics and potential bad PR of that ("Google are taking over Firefox") were not worth it for them.