Yeah I suppose there probably are a pretty large number of companies that force their employees to use only certified browsers and browser versions.
Additionally, there may be some site that they use daily for work purposes that only functions properly in an old browser version. That is the case for many government websites. I have to input and change data on government servers daily, and their site only functions fully with internet explorer (for example).
It's probably just not worth it to them to have two browsers open, so they just keep the one open that works with the site they frequent. excellent point Dara.