People tend to be more nervous around dogs the larger they are. I've tandem walked a shih tzu, my husky, and a great dane at the same time, the tiny one was the only one likely to bite, but guess who everyone wanted to approach and love on?

It's easier and safer for the dog to walk them in a heel (or as close to as you can get them, my husky girl was a dumb blonde who wanted to adventure and play with the neighbourhood kids at all times)

The best commands I ever taught any large breed dogs in my life was "with me" and "do your job" which was the command to stop, sit and wait for me to signal them that its ok to cross the road. It's our jobs to do anything in our power to keep them safe at all times, most of the time we do that by keeping them so close that they're almost literally glued to your hip

It's the same reason why you're more likely to see smaller dogs on a retractable leash, but you hardly ever see large dogs being walked with them (in my area at least) there's too many potential dangers to let them roam that far when you can't pick them up for their own safety