Why do you think so many jobs that could be done sitting down require people standing for hours on end?

Like cashier's for example, some stores allow their cashier's to sit and do their job, but most retail stores require those workers to stand for up to 8 hours which does cause physical harm to their bodies. Another example of this would be security workers, some are allowed to sit but others are made to stand there, both are able to do their job but one is in considerable pain.