This is too good a train of thought not to board.
But what reason would you have to conclude this, whether emotionally or not? What would lead you to conclude that an arrowhead was designed but that, say, a cube of salt or a crystal of quartz, was not designed?
I know what would lead me to conclude this. Extraneous evidence. Just a bunch of arrowheads sprinkled willy-nilly across a field, or a continent, wouldn't do it. There are other bits of evidence that have been discovered that lead one to the conclusion that arrowheads were designed.
- Other chunks of flint that match up seamlessly with arrowheads.
- Tools that show signs of being used to flake off bits of flint.
- Arrow shafts and the binding cord used to tie the heads to the shafts.
- Verbal records of how to make arrowheads.
And so on.
It's all this other stuff that we know about arrowheads that we don't know about salt or quartz that lead us to conclude that arrowheads are designed. Without this evidence, there would be no reason to believe that arrowheads were designed.
And this is exactly the kind of evidence we don't have about the universe. We don't have evidence of the raw materials, or the tools, or the passed-down instructions (we have stories, but not instructions) of how to make a universe.