Probably for the same reason Apple and Google both produce their own browser - it's too important a part of the user experience to delegate responsibility for it to a third party.
It's unlikely Google would suddenly turn around and say they're no longer producing Chrome for Windows, for example, but it's not impossible.
Also, IE, or rather Trident, is a core OS component. Applications rely on it for displaying web pages and HTML. Applications can also be written in HTML and JS, both HTML Applications (since... Windows 98, I think?) and WinRT applications (since Windows 8).
Yeah you're probably right. They want the experience to be cohesive with the rest of Windows. Last time I looked Chrome still doesn't have a metro mode besides just opening up in full screen mode, so there's one reason to keep IE around
It's unlikely Google would suddenly turn around and say they're no longer producing Chrome for Windows, for example, but it's not impossible.