Why doesn't python concatenate string literals with adjacent string variables?

Question

I'm curious why this works:

>>> "foo" "bar"
'foobar'

But this doesn't:

>>> str1 = "foo"
>>> str1 "bar"
    str1 "bar"
         ^
SyntaxError: invalid syntax

I understand I can use a +, ''.join(), etc. to concatenate a string variable. More asking out of curiosity.