Why can't I use a starred expression?

It's because this:

(a)

Is just a value surrounded by parenthesis. It's not a new tuple object. So your expression:

>>> '%d %d' % (*a)

will get translated to:

>>> '%d %d' % * a

which is obviously wrong in terms of python syntax.

In order to create a new tuple, with one expression as an initializer, you need to add a ',' after it:

>>> '%d %d' % (*a,)

Note: unless a is a generator, in this particular situation you could just type:

>>> '%d %d' % a

Also, if I may suggest something: you could start using new-style formating expressions. They are great!

>>> "{} {}".format(*a)

You can read more about them in those two paragraphs of python documentation, also there is this great website. The line above uses argument unpacking mechanism described below.

Update: since python 3.6, you could also use string interpolation - f-strings! These are described in PEP-498, and some examples can be found in Python documentation.

Starred Expressions

There are many more uses to starred expression than just creating a new list/tuple/dictionary. Most of them are described in this PEP, and this one

All of them come down to two kinds:

R-value unpacking:

>>> a, *b, c = range(5)
# a = 0
# b = [1, 2, 3]
# c = 4
>>> 10, *range(2)
(10, 0, 1)

Iterable / dictionary object initialization (notice that you can unpack dictionaries inside lists too!):

>>> [1, 2, *[3, 4], *[5], *(6, 7)]
[1, 2, 3, 4, 5, 6, 7]
>>> (1, *[2, 3], *{"a": 1})
(1, 2, 3, 'a')
>>> {"a": 1, **{"b": 2, "c": 3}, **{"c": "new 3", "d": 4}}
{'a': 1, 'b': 2, 'c': 'new 3', 'd': 4}

Of course, the most often seen use is arguments unpacking:

positional_arguments = [12, "a string", (1, 2, 3), other_object]
keyword_arguments = {"hostname": "localhost", "port": 8080}
send(*positional_arguments, **keyword_arguments)

which would translate to this:

send(12, "a string", (1, 2, 3), other_object, hostname="localhost", port=8080)

This topic has already been covered to a substantial extent in another Stack Overflow question.

My question, why?

Because your python syntax doesn't allow that. It's defined that way, so there's no real "why".

also, it's unnecessary.

"%d %d" % a

would work.

So, you'd need to convert your expansion to a tuple – and the right way of doing that would be, as pointed out by Lafexlos, be

"%d %d" % (*a,)