Wikipedia says
When using Microsoft Windows, the standard Italian keyboard layout does not allow one to write 100% correct Italian language, since it lacks capital accented vowels, and in particular the È key. The common workaround is writing E' (E followed by an apostrophe) instead, or relying on the auto-correction feature of several word processors when available.
As you seem to be interested in writing in Italian with that already has a different set of characters written on it, I recommend you to use a different layout where you have dead keys for diacritis, such as a Portuguese or Spanish layout.
PS: I cannot understand the reason, either. Specially when already providing an Alt Gr key with little use. (On Linux you will probably find many undocumented Alt Gr combinations adding extra characters).
