Why don't we have some Proto-Slavic words in Proto-Indo-European?

I think that I worded my question badly...

Anyways. When I was searching for etymology of the word "obraz" I just found Proto-Slavic "образъ". And then I looked for etymology of otac and I again just got proto-slavic "отьец". Why is it? Did they just make words out of nowhere or do we just not know where did these words come from?