Horns were not fully/correctly formed at birth. As such, hornless.
I mean, I'm pretty sure qunari models have protrusions where horns go, as shown in that hornless pic.
They would have to redesign all qunari or make a special model for the hornless most people are complaining about. Not worth the effort really.
In most cases of a hornless qunari, they were. This modification is in fact very popular among Tal-Vashoth. (This, by the way, is probably where the bald spots Ryzaki was complaining of came from. Another reason to give any Qunari I roll horns.)
This is my favorite Qunari concept art:

I will grant that their hair COULD be covering horn nubs, but even the more concealing hairstyle available in DAI doesn't completely hide them from view.
Also... they already made a special model for hornless Qunari - they made the nubs! It would have SAVED them some time and effort to make the special model without the nubs instead.