Sure, and you can do that 20/30 minute walk if you want, there are many parts of minnesotan suburbs that are, in fact, very walkable already. On a weekend, that is a nice thing to do - but the day-to-day life that the majority of people live shouldn't be optimized for that.

I'm not sure why you're shoe horning body weight into this - that's a whole separate can of worms that tenuously related, but not relevant to the fact that these places are so spread out in such a way that walking isn't feasible for a myriad of other very practical and immediately relevant reasons (weather, ability to organize child care/education, ability to run errands before/after work, time spent "commuting", etc.)

You don't get it, it's not "optimizing" anything.

In a lot of places it's close to impossible to do what you're saying. There are no side walks. Many suburban streets and especially those bigger roads (stroads) are horrible. No shade because no trees because HUGE ADS SHALL BE VISIBLE FROM CARS, lots of dangerous driveway exists every 5 minutes that you can't even walk in peace lest you are run over by a huge truck, etc.

Streets are dangerous for cyclists (and I mean the regular cyclists, commuter/grocery shopping style, not the lycra-clad racers).

There are modern ways to design infrastructure, it isn't even a lot more expensive than the old fashioned way, and it makes for a lot more pleasant environment for everyone. Even drivers get to enjoy it because... people start walking (under 1km) and cycling (under about 5-7km), so a lot of car traffic just vanishes. So the remaining car drivers get to vroom-vroom a lot more :-)