I've already explained why many or most people would willingly give up their ability to help themselves. Why you in particular might choose (or not) to do so isn't relevant as to whether it's something that should be designed into and sold with every vehicle on the road. If I have a flat then, depending on the situation, I'm likely to change it out. My mom? Not so much. In fact, most of the people I know would be scared to try it.
As I clearly said, you are free to buy your own spare. Most people rarely leave metropolitan areas and, when they do, they are usually on main arteries with lots of nearby infrastructure.
From a risk perspective, it's a whole lot more dangerous to drive without a good first aid kit, blankets, and water. If anything, that's what should come standard and be refreshed as regular maintenance.