Why Aren't There Better User Agent String Standards?

[Vulcan]

I know why IE starting using the Mozilla stamp, but why does it continue to use it?

Why don't the browser teams come to together and come up with a better standard?

For example:

IE/9 (Windows 7; 64bit​

Instead of:

Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Win64​

Or

Firefox/4.0.1 (Windows 7; en-GB​

To replace:

Mozilla/5.0 (Windows; U; Windows NT 6.1; en-GB; rv:1.9.2.17) Gecko/20110420 Firefox/4.0.1​

 

[Vulcan]

Because they don't know how much they'd break by changing it.

What would break?