There is a huge difference between the book Sharpe's Gold and the film, but the screenwriter did have a problem. As you may know Paul McGann was first selected to play Sharpe, but had a dreadful accident while filming and was unable to continue (he's now happily recovered). The production was forced to make a huge insurance claim (and replace Paul with Sean Bean). They had been working on Sharpe's Gold - but on a script that followed the book much more closely. Legal advice told them they could not film that original script because the insurance company might then claim that their losses had not been so grievous, so a new (and weird) script was devised.
I honestly have no idea. It's a question that should probably be directed to whoever wrote those scripts. I take no part in the writing and try to keep a big distance between myself and the production!
Because the script-writer wanted that! I have to emphasize that I'm not involved and, moreover, don't want to be involved. I have my work cut out writing books and don't want to be a part-time interferer in the TV production!
The real intention was to write Hornblower on land, and Sharpe is really modelled on Hornblower . . . not in character, but in genre. Bond, I think, is more fantastic, and while I'm not claiming that Sharpe is a realistic picture of a Napoleonic soldier, I do try to make the background of his adventures as realistic as possible.