Because getting from max hot to max cold means pressing a button up or down for each degree. And there is lag between the car temp sensor detecting the current temp and the change in air temp and fan speed. On top of all that, the human body doesn't have a thermostat. What feels comfortable depends on your internal body temp, humidity, how sunny it is, what activities you've just done or will do, what you're wearing, and how many other people are in the car. This information cannot be summarized in a single number. It doesn't give you what you want automatically, and you need to tweak it up and down anyway.

Instinctively, you know exactly whether you want hot or cold air and how strong you want it to blow. I could either turn two knobs to get exactly what I want, or I could press buttons to guess at the temperature that is going to trick the automated systen into giving me what I want without knowing if I will need to make additional adjustments later. Worse still, making adjustments means taking my eyes off the road to see what number I've put into the system. This problem is compounded exponentially if the car has a touch screen and I need to navigate menus to adjust the AC.

The worst part about it is that it's entirely unnecessary. The knobs worked fine. Some older cars has sliders ir buttons, but knobs seemed to be the thing everyone liked, and the automated system didn't improve on that. You still have to adjust the air, but the interface to make adjustments is objectively worse.