I work for a major European corporation and I am also stuck on XP. We don't exactly have the best in class IT dept (despite claims by a senior IT guy that we have more developpers than google and facebook together... we don't really have much to show for it).
I never understood why users stuck on XP don't just install a different browser (Chrome or Firefox). You don't have to use IE, especially not in the version is so old it barely works with modern sites.
Saying that doesn't actually answer the question, it just shifts it to "why would corporate policy say that".
That's probably why you can't get anything done (Mythical Man-Month passim)
[edit] in fact if anything third party software is forcing us to upgrade as we see some critical third party software progressively becoming incompatible with XP.