1) virus def updates rarely cause any problems whatsoever. Whereas browser updates are actually just the opposite. At enterprises its not uncommon that even dot releases break some apps. So those apps need to get fixed, before wide rollout.
2) Virus def updates, when they do break people, break a LOT of people very visibly. That is, if you wait a day on a virus def update, you can pretty quickly see if it broke ppl just by doing an online search. Whereas with browser updates, its a lot more difficult to tell if any of your apps will be broken with a given update.
3) Virus def updates are often fixing issues that have the potential to directly effect your business. Whereas, except for security fixes in browsers, browser updates won't have any effect on the business. It's rare that you have an app waiting to be deployed, but you're waiting on the next version of FF for a new feature or bug fix. IOW, you're taking risk, with no upside.
It's really just not the same thing at all.