Nothing about that is a solution or simple.
It may not be feasible in all cases, but it's definitely feasible in some cases, and it absolutely works.
In particular, bike commuting is an excellent way to integrate exercise into your daily routine without going to the gym or the tedious exercise-for-it's-own-sake activity. It's not practical for everyone, but between power-assisted e-bikes and the observation that you don't need to do it _every_ day, it's a lot more achievable than many think.
Similarly, if you take public transport to work (or elsewhere) you could get off a stop or two early and walk the rest of the way to get some of the benefits even if lacking the time or stamina to do the whole trip on foot.
Integrating exercise into some task or activity you were going to do anyway (e.g., commuting or running nearby errands, if applicable) is much easier than trying to maintain discipline and motivation to exercise as a dedicated, stand-alone activity.
Surely somewhere in that more-than-half-of-all-car-trips there are a few examples of car errands that could be easily replaced by bike, scooter or foot.
EDIT: incidentally even in the most car-centric dystopian suburb there are ways to incorporate more walking into your routine. The next time you go to a strip-mall or big box store you could park on the far side of the lot and walk to the door. A minor inconvenience in both time and effort, but you could probably add 1/2 mile or more of walking to your Saturday afternoon errands that way.
But I stand by the statement that most able-bodied Americans could find some way to incorporate more human-powered transport into their routine if they really wanted to. And in many cases they could do it without any adding any superfluous travel. I.e., it may be slower or even less convenient in some way, but it was a trip you were going to make anyway. You're just changing the mode of transit.
(Incidentally if it's truly too dangerous or inconvenient to walk across the Costco parking lot with a loaded shopping cart I think we're well on our way to the people from Wall-E.)
Like yeah, they maybe can't, are you disabled? No? Well then.
It is nevertheless eminently possible to walk and bike more.