It's because our whole society is built around having a car. There is no real alternative. Taking away a license inflicts a major burden on them. The idea that "driving is a privilege, not a right" is a cruel joke.

I had a friend who's mom had a seizure while driving. Her foot jammed the accelerator to the floor while going downhill. She didn't hit anyone, and survived the inevitable crash, but there wasn't much left of the car. It was likely going to happen again, and yet the state did not take away her license. Now, the family was responsible and made sure she didn't have to drive anymore, but not everyone has a responsible and proactive family like that. There are almost certainly people with similar medical issues driving around right now, and it keeps me awake at night. Not even an uncontrollable medical issue that makes you a rolling time bomb is enough.