Three reasons for witnesses standing: they are easier to hear, for people find it easier to speak more loudly when standing; the witnesses' 'body language' and demeanour is read more easily and more fully; the witness box has a ruddy great high front to it and short witnesses would disappear sitting down!
But some judges still get quite carried away by genteel etiquette to women. They sometimes almost rush out to offer the poor, weak, fainting things a chair! "Do sit down, if you feel more comfortable, madam' they purr, at the very least.
Americans manage with an open-fronted stand, but we prefer to hide the legs of our witnesses. They don't have a dock, either. Look, it's bad enough having the lay client passing occasional notes, usually both irrelevant and distracting, from the back via the solicitor's clerk. Having the client sitting right next to you and talking would be hell! It would only encourage them