Because it's the near-equivalent of misspelling your name on an exam, at least when it's entering your own email address.
On the flipside I sometimes get mails intended for someone else with my name in Australia, but that is easy enough to assume that his friends/kids' teachers/etc. just manually typed in the address and messed it up.
I got added to a Black Board announcement list for some school. They don't seem to have a way to say 'stop sending this address mail'. I'm cranky, so I emailed the dean of student affairs about it.
He emailed back as if I were a student. I responded pointing out that I had no association with the school. He emailed back asking me to explain further, as there was someone with my name at the school. So I explained that I thought it was probably someone with the same name. At least that ended it.