Why doesn't Ruby support i++ or i--​ (increment/decrement operators)?

The pre/post increment/decrement operator (++ and --) are pretty standard programing language syntax (for procedural and object-oriented languages, at least).

Why doesn't Ruby support them? I understand you could accomplish the same thing with += and -=, but it just seems oddly arbitrary to exclude something like that, especially since it's so concise and conventional.

Example:

i = 0    #=> 0
i += 1   #=> 1
i        #=> 1
i++      #=> expect 2, but as far as I can tell, 
         #=> irb ignores the second + and waits for a second number to add to i

I understand Fixnum is immutable, but if += can just instanciate a new Fixnum and set it, why not do the same for ++?

Is consistency in assignments containing the = character the only reason for this, or am I missing something?