They can't, many businesses rely on Microsoft's browser lockdown support. For example strict IE from browsing through the local disk or restricting settings from being modified. Firefox and Chrome have virtually no lockdown support.

Can't you secure everything on the operating system level?

I am sorry, but adapting to a vendor is not how IT departments run (especially when a vendor aims to replace your IT departments).

They do when they don't have a choice (which is also mostly the only way you can get most people to change at all).

I think a lot of places would drop google apps before they let google dictate what software they use.