Simply because [ 1 ] the key is not in the map and [ 2 ] those two operations are different in nature.
From dict Mapping Types:
d[key]
Return the item of d with key key. Raises a KeyError if key is not in the map.
get(key[, default])
Return the value for key if key is in the dictionary, else default. If default is not given, it defaults to None, so that this method never raises a KeyError.
