No matter how much you push and say that everything Slack offers is doable in IRC, doesn't change the fact that you still have to go through the troubles of setting that stuff up.
While I agree that setting up a bot isn't hard, I've done it for the IRC channel I created and still sit in daily, but fact of the matter is Slack is a matter of signing up and enabling a couple of features.
My job for my company is to build our product, not an internal chat tool. That's what Slack is for.