... But seriously, for the same reason you refer to 'x' in Python like it's already defined. It's just a symbol.
"I recently bought an iPhone 4 for my wife Emily"
"I recently bought an iPhone 4 for my daughter Emily"
"I recently bought an iPhone 4 for my dog Emily"
>>> Emily Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'Emily' is not defined