Most folks don't do retail for fun. Those that do will also do a number of other part-time jobs for fun or whatever reason. Most folks working retail, though, are doing it to eat and pay rent. Sometimes having a flexible schedule is necessary. A great number of places have chances to move into management, sometimes moving out of poverty wages.
None of this means the folks aren't ambitious or motivated. You simply don't recognise it as such because they are doing what you consider to be a "lower" job or not doing enough to meet your personal criteria.
I will grant you that selling garden soil might be as physically demanding as working in a factory, and miles walked could add up too. But I never woke up in retail wondering if I'd be physically able to continue working my job much longer because of the wear and tear.
There's a reason every rich country got there by manufacturing and not retail, and in part it's that you don't build human capital by sitting in a chair scanning price tags. Manufacturing productivity increases steadily so workers have to become steadily more skillful to keep up. It's also far from a dead-end job, as productivity is easily measured and the cream can rise.
People in retail can be ambitious and talented, but their talents are wasted there. We can't get every ambitious, motivated person out of retail and into the job where they would do the most good just by offering them money, because money isn't everything. But we can avoid screwing things up by _pushing_ those people into jobs where their talents are wasted, by offering them too much money. I've worked lots of mandatory overtime because people wouldn't come work for my employer, so I feel the pain of jobs that need good people but can't get them.
I don't like interacting with people but it is way more cushy than the factory work I did for fifteen years. I guess you're right about not taking more intelligence or training, but it takes so much more physical stamina and risk of accident.