I believe for the majority of people, that just isn’t feasible for reasons of money, location, limited transportation choices, stuff like that. It seems it would be really difficult to transport all the material required to live off the land, if there is limited or no fuel available to move. Things like tools, storage containers, etc, are heavy and take up a lot of space.
I would also agree that one cannot feasibly carry all those things that would be needed in escaping from a high density area to a rural/country area, but that's where the prepping thought process begins.
Some citybound folks have gone so far as to buy a separate bugout piece of property in the country and have living facilities (camper, trailer, vacation cabin) there already stocked with the essentials. But like you said, most folks may not can afford that option. Hopefully, in the alternative they have family or friends that live in a rural area that have all the essentials at their household so that whatever you can bring, whether it's additional food, medicines, silver, ammo, etc. will at least help add to the stockpile of goods. Again, that's why it's so important to have that group that can rely on each other, to lessen the burden of each individual from having to provide everything one needs.
Sorry to get so verbose. It's an important issue with me.