My answer is probably/hopefully out of date.
It's because it's being made by a cashier, not a food maker.
At the Golden Arches, you have some wonderful people who are committed to making your food well and quickly. They're usually making sandwiches, meat, or fried products.
Then you have lazy bastards like me who are working the headset and window position. After minutes of tapping the screen without thinking, filling drinks, and cashiering, having to do anything involving ice cream just completely breaks the rhythm.
I may have been reluctant to do it, I may have been able to get someone else to do it. I may have made the thing and gotten ice cream on my face.
I had an office job years later where every single time I touched the metal doorknob, I would get a static electrical shock. I still hesitate when touching doorknobs today, because I got weirdly shocked so many times.
Mixing up the ice cream products reminds me of a relatively harmless albeit annoying version of that. Getting sticky crap in your eyelashes will make you hesitate to do it in the future.
I apologize on behalf of all fast food employees who have weirdly pretended to be better than you and made you wait. They just don't have the same work ethic as the people making most of the rest of the food.