According to ustr.gov, the US economy is 68% service. Factory jobs are not part of the service economy, and according to that statistic, are rare. Even within the 30% non-service, a lot of jobs require standing up, such as working on a farm. Plants are closing across the country because of lower-regulation and cheaper-labor countries. Furthermore, factories are moving towards higher education jobs as technology increases. Unless you live near one of these factories, you're working at Walmart if you don't have a college degree. This American Life interviewed people who couldn't even think of jobs which didn't require standing up, because they were in the "working" class and had very little education.
Citation: Bureau of Labor Statistics Data
Compare this with retail:
Note that the population is rising by about 25 million for the period in these graphs, making the drop even more powerful.